Duplex printing with card stock paper jams in back on Brother MFC-9840CDW

Brother printer not printing double sided correctly
If the Paper Adjustment Lever for 2-sided(Duplex) Printing is not correctly set for the paper size, paper jams may occur and the print will be out of position on the paper. Open the Properties dialog box in the printer driver. ... The printer will print on both sides of the paper automatically.Sep. 2, 2016

Print on both sides of the paper(2-sided/Duplex Printing ...

https://www.google.com/search?q=Brother+Printer+Duplex+Not+Working

Your machine should be able to handle a 110gsm card stock. In your computer printer settings you should be able to change the paper type to either card or heavy/ thicker paper. If the card stock is quite stiff your machine will not be able to bend it around the paper feed roller regardless of your print settings.

remove the small, or scratch into pieces inside the duplex to clear the paper jam.

Check the C6200 Device Settings in the properties and make sure that the Duplex Unit is installed.

this will be an issue with the duplex rollers check where it is jamming is it in the duplex unit if yes then check the duplex rollers for wear worn rollers will cause scews also check there is no torn pieces of paper stuck in the guides of the duplex
a good thing to do is to get a thin piece of card stock slide it into the duplex manually and frred it where the paper path goes on the duplex from beginning to end and it will push out any thing in the path

I just returned my HL-5370DW printer to Staples. I only had it for 3 days and found it running hot, warping card stock and envelopes, and jamming regular size paper constantly. It appears to be OK for very small print jobs but not very robust. I did like the duplex printing and the price. I do not believe there is a solution other than replacing this printer.
Xelzbear

Your pickup tires on the printer may be worn, how many prints have you made on this printer. it may be time for a maint. kit.

http://www.brother-usa.com/ModelDocuments/Consumer/Software%20Users%20Manual/SUM_DCP_7030_7040_MFC_7340_7345N_7440N_7840W_EN_1650.PDF

here is the manual p4 and p16 discuss the topic:
Duplex printing 1
The supplied printer driver supports duplex printing.
Guidelines for printing on both sides of the paper 1
�� If the paper is thin, it may wrinkle.
�� If paper is curled, turn over the stack, straighten it and then put it back in the paper tray or the Manual Feed
Slot.
�� If the paper is not feeding properly, it may be curled. Remove the paper, straighten it and turn over the
stack.
�� When you use the manual duplex function, it is possible that there may be paper jams or poor print quality.
If there is a paper jam, see Paper jams in the User’s Guide. If you have a print quality problem, see
Improving the print quality in the User’s Guide.
Manual duplex printing 1
The machine prints all the even-numbered pages on one side of the paper first. Then, the Windows® driver
instructs you (with a pop-up message) to reinsert the paper. Before reinserting the paper, straighten it well,
or you may get a paper jam. Very thin or thick paper is not recommended.
Simultaneous scanning, printing and faxing 1
Your machine can print from your computer while sending or receiving a fax in memory, or while scanning a
document into the computer. Fax sending will not be stopped during PC printing. However, when the machine
is copying or receiving a fax on paper, it pauses the PC printing operation, and then continues printing when
copying or fax receiving has finished.


�� Manual Duplex
Check Manual Duplex. In this mode, the machine prints all the even numbered pages first. Then the
printer driver stops and shows the instructions required to re-install the paper. When you click OK the odd
numbered pages are printed. There are six types of manual duplex binding directions available for each
orientation.
�� Booklet Printing
When you want to print a booklet, use this function. To make the booklet, fold the printed pages down the
middle.
�� Binding Offset
If you check the Binding Offset option, you can also specify the offset for binding in inches or millimeters

Were you able to remove all of the original jammed business cards? where the leading edge of the "Fanfold" prints occurs, you will probably find something blocking the paper path. HP printers usually have a limit on the paper weight that the printer can handle when doing duplex (2 sided) printing. Check this against the weight of the cards you are printing.

HI there sounds like there is a blockage in the duplex area that is not holding any sensors down. Check the duplex area for torn paper, failing this it maybe a sensor problem which will then need a engineer.

Hope this helps
Thanks J