Poor printing quality. Colors not showing up, white line

For severe print quality/missing color issues there could be an air bubble/air gaps in the printer ink lines or a portion of the PRINTHEAD is dry, so you should first perform a Clean PrintHead procedure from your printer control panel.
NOTE: If one or more the colors were previously dry fired during printing (i.e. if you continued printing using only a few colors to one, air may have been accidently ingested into the ink lines between the cartridge(s) and PRINTHEAD. To recover from this problem, you may have to run the Clean Printhead procedure 2-3 times.
If a problem persists then it is suggested to review the steps outlined on the following HP 920 support pages, including printhead removal/printer cleaning and service station cleaning. It is suggested you refer to printer manufacturer instructions first as the Inkjet411 procedures are provided for general guidance purposes only. A dirty service station can cause poor image quality and/or ink cross-contamination between different colors:
http://inkjet411.retailinkjet.com/?page_id=1495
If the problem still cannot be corrected, then the PRINTHEAD may be dry and may need to be flushed (i.e. with warm water as it could have dried out, but this is a function of how much you print). Remove and soak in hot water for 5 minutes...thoroughly dry printhead prior to re-installing. Retest.
For more information please visit:
http://inkjet411.retailinkjet.com/?page_id=856
Worst case scenario the PRINTHEAD may simply be electrically damaged or at end-of-life (typically fails between 3,000 and 5,000 printed pages) and may require replacement, usually accompanied by error messages like Incompatible Printhead or Ink Delivery/System Failure. NOTE: The PRINTHEAD will eventually fail in time. For more information on the how the PRINTHEAD works, watch the video here: http://inkjet411.retailinkjet.com/?page_id=1983
To verify Printhead performance, run the Tap 41 diagnostic test, (Pen Status: Pen OK).....for printer model instructions please visit:
http://www.Inkjet411.com

1. First, check your cartridges. HP cannot guarantee the quality of remanufactured or refilled cartridges.
2. Second, make sure your paper is not causing the problem. Paper that does not accept ink well can cause your images to smear or bleed. Plain paper is not the best choice for high-density graphics. Click here to see how to pick the best paper for your project .
3. If the paper is not the problem and you are using genuine HP cartridges, print a clean test to see if the printer and cartridges are functioning properly. Load the printer with clean, plain, white paper and print a clean test page. To see how to print a clean test page, see your printer's User's Guide , or see HP Deskjet Printers - How To Print Internal Tests , or HP All-in-One Product Family - Printing Self-Test Reports .
4. Evaluate the clean test page. Different printers produce different pages, but you should recognize elements from the examples below on your test page. In general, cartridge and printer problems are indicated by streaks, missing colors, or wrong colors.
   • Examples from a good printer and cartridges (your test page may not look exactly like this): Figure 1: Solid colors that match color dots on cartridge label Figure 2: Pattern unbroken, or broken in only a few places
     
     If your test page looks more like the good examples above than the bad examples shown in the next two sections below, the printer and cartridges are probably not causing the problem. Click here to continue troubleshooting .
   • Examples from a failing printer or cartridges - streaked or broken bars or lines Figure 3: Fading, missing, or streaked bar - low-ink or empty cartridge, clogged nozzles, or electrical problems
     
     
     
     
     
     Figure 4: Color completely missing (magenta bar missing below) Figure 5: Lines broken randomly - clogged nozzles
     
     Figure 6: Lines broken in pattern - dirty contacts or other electrical problems
     
     
     
     Figure 7: Color completely missing (magenta pattern missing below) If your test page looks like these examples, you probably have a cartridge that is empty, has clogged nozzles, or dirty contacts. Click here to continue troubleshooting .
   • Examples from a failing printer or cartridges - colors do not match the color dots on the cartridge label (Note: do not compare the test page colors to the colors of the examples on this web page. Your monitor may not display the colors properly). If your test page looks like these examples, you probably have a cartridge in which the inks have mixed. Click here to continue troubleshooting .

Found this on the web. Click on link below to print the entire manual out to keep. Below are steps to do cleaning. If you find this helpful pls rate me. Thanks and Good Luck.

http://www.brother-usa.com/ModelDocuments/Consumer/Users%20Manual/UM_MFC_210C_420CN_EN_286.PDF

How to improve print quality Cleaning the print head
To ensure good print quality, the MFC will regularly clean the print head. You can start the cleaning process manually if needed. Clean the print head and ink cartridges if you get a horizontal line in the text or graphics on your printed pages. You can clean Black only or three colors at a time (Cyan/Yellow/Magenta), or all four colors at once.
Cleaning the print head consumes ink. Cleaning too often uses ink unnecessarily.
Caution
Do NOT touch the print head. Touching the print head may cause permanent damage and may void the print head’s warranty.
1 Press Ink.
2 Press ▲ or ▼ to select Cleaning.
Press Menu/Set.
3 Press ▲ or ▼ to select Black, Color or All.
Press Menu/Set.
The MFC will clean the print head. When cleaning is finished, the MFC will go back to standby mode automatically. If you clean the print head at least five times and the print has not improved, call Brother Customer Service (see Brother numbers on page i).
Checking the print quality
If faded or streaked colors and text appears on your output, some of the nozzles may be clogged. You can check this by printing the Print Quality Check Sheet and looking at the nozzle check pattern.
1 Press Ink.
2 Press ▲ or ▼ to select Test Print.
Press Menu/Set.
3 Press ▲ or ▼ to select Print Quality.
Press Menu/Set.
4 Press Color Start.
The MFC begins printing the Print Quality Check Sheet.
5 Check the sheet to see if the quality is OK by following these steps:
Color block quality check
The LCD shows:
6 Check the quality of the four color blocks on the sheet.
7 If all lines are clear and visible, press 1 (Yes), and go to Step 10.
—OR—
If you can see missing short lines as shown below, press 2 (No).
The LCD will ask you if the print quality is OK for black and three colors.
OK Poor
Is Quality OK?
1.Yes 2.No
Black OK?
1.Yes 2.N
8 Press 2 (No) for Black or three colors
with a problem.
The LCD shows:
Press 1 (Yes). The MFC will start cleaning the print head.
9 After cleaning is finished, press Color Start. The MFC will start
printing the Print Quality Check Sheet again and then return to
step 5.
10 Press Stop/Exit.

Shaking these cartridges does nothing except give you a little exercise. If this is a brand new HP cartridge you are having problems with I would return it to the store you purchased it from. HP brand cartridges should not give you these problems right out of the box. The print defects you are seeing are due to a dried up or clogged print head. If you like you can try hydrating the print head on a water soaked paper towel until the ink starts flowing, but there's a risk you will cross contaminate the colors.

Check the following information please.

General print quality issues

Solving issues with color documents This section describes issues that can occur when you print in color. Color document issues Problem Cause Solution Only black is printing when the document should be printing in color.

Color mode is not selected in your program or printer driver. Select color mode instead of grayscale mode. The correct printer driver might not be selected in the program. Select the correct printer driver. The device might not be correctly configured. Print a Configuration page.

One or more colors are not printing, or are inaccurate.

Sealing tape might be on the print cartridges. Verify that the sealing tape has been completely removed from the print cartridges. The media might not meet HP specifications. Use a different paper, such as high-quality paper that is intended for color laser printers. The device might be operating in excessively humid conditions. Verify that the device environment is within humidity specifications. One or more print cartridges might be low. If you are using non-HP print cartridges, no messages appear on the device control panel or in the HP Toolbox. Replace any low print cartridges. The control panel is set for override. Replace low or empty print cartridges. A color is printing inconsistently after you load a new print cartridge.

Another print cartridge might be low. If you are using a non-HP print cartridge, it might be low or empty but you will receive no indication. Remove the print cartridge for the color that is printing inconsistently and reinstall it. The control panel is set for override. Replace low or empty print cartridges. The colors on the printed page do not match the colors as they appear on the screen.

The colors on your computer monitor might differ from the device output. Select Screen Match in the Color tab of the printer driver. If extremely light colors or extremely dark colors on screen are not printing, your software program might interpret extremely light colors as white or extremely dark colors as black. If possible, avoid using extremely light or extremely dark colors. The media might not meet HP specifications. Use a different paper, such as high-quality paper that is intended for color laser printers. The finish on the printed color page is inconsistent.

The media might be too rough. Use a smooth paper or print media, such as a high-quality paper that is made for color laser printers. Generally, smoother media produces better results. Repetitive image defect ruler Use a repetitive image defect ruler to measure occurrences of repetitive image defects to help solve image quality problems. Place the ruler next to the first occurrence of the defect on the page. Find the distance between identical defects and see the Repetitive image defects table to identify the component that is causing the defect. Repetitive image defects Measurement Component 24.7 mm0.97 inch Developer sleeve 26.7 mm1.05 inches Charge roller 32.9 mm1.3 inches RS roller 37.7 mm1.48 inches Transfer roller 54.5 mm2.15 inches ETB rollers 56.6 mm2.22 inches Fuser sleeve 62.8 mm2.47 inches Pressure roller 75.4 mm2.97 inches OPC
Have a good evening

make sure the cartridge is firmly seated in the carriage. If so you may just have a defective cartridge. Return it to Walmart and exchange it as a defective cartridge otherwise.

My daugther also has a Pix780 so I just replaced my print head with hers and now the colour prints great. I guess I'll try cleaning mine first and if that doesn't work, I'll purchase a new one when the dealer who services Canon opens on Tuesday. I could also buy one from the Canon website for something like $60. I copied the following from Canon's sites under the FAQs. Sounds like you may need to replace your printhead.

How does a Canon Printhead work?
The printhead delivers very small drops of ink onto the paper. The ink from the cartridge is transported via small tubes. Heating elements heat the tubes, creating a small bubble of ink. The heating element determines the size of the drop to be applied to the paper.

How do I know I may need to replace my printhead?
There are a few symptoms that may indicate printhead failure or a clogged printhead. If the following symptoms continue even after replacing the ink cartridges and trying the troubleshooting steps in your user's manual or those listed below, you may need to replace your printhead:

• Pages come out blank, or you see poor print quality
• Pages you print appear to be missing colors
• Colors on the printed page are quite a bit different from what shows on the screen
• Horizontal white lines or light-colored lines or light streaks appear throughout the page

Why would the printhead have problems?
After hours of printing, the heating elements may fail after being turned on and off hundreds of thousands of times. The printhead can also become clogged or blocked with paper debris or solidified ink. If your print quality is poor or your printer will not print at all, it may be time to change your printhead.

sounds lke your yellow nozzle not working properly...had same issue on lexmark printer, ended up geting this epson...learnt from my mistakes...perhaps it ran dry? or you left the yellow cartridge out longer than the rest....only you might know what happened

if you replace the black ink cart with an original one then it's the print head
Solution: Cleaning the Print Head - C555 / C560 / C635 / C530 / C545

Untitled Document Follow this procedure to clean the BJ cartridge print head:
1. Press 2. Press 3. Use or to select the [HEAD CLEANING]..
4. Press .
The MultiPASS cleans the print head.

The cartridge needs cleaning. There are several things you can try. Using a wet paper towel, you can wipe the gold portion of the cartridge to try and draw out ink, or you can dunk the entire cartridge in warm water for a couple minutes or so (I have done this with success). The process is more clearly explained here:

http://www.usainkcartridges.com/faq.html