I'm getting white streaks (weak printing) on my ricoh c420dn. It happens only with yellow and cyan colors as you can see in my orkut (public) profile...
There are many possible causes. You'll have to do a bit of detective work to find your particular problem. you're either getting unwanted toner, or the printing process is being interrupted.
The photosensitive drum can get a mark or a spot or even a damaged area. This usually presents as a regular area which repeats, going down the print, rather than a streak It may be possible to clean the drum with a soft tissue -- perhaps with the addition of a little Isopropyl alcohol. If you have a replacement drum, try that. In most cases the drum is part of the cartridge, but not always.
Loose toner in the system.
If your drum is leaking, or you've spilled some toner inside the printer (perhaps after shaking the cartridge to get more life out of it) the excess toner may cling to the paper in streaks and get fused onto it. The cure is a good vacuum clean of the insides.
Sometimes a small corner of a sheet of paper (or similar) can get caught in the workings of the printer, This can obscure part of the optical (especially with LED printers) and cause a blank streak in the printing. Dust and toner on the LED head can cause similar problems and fuzzy printing. Tissue and alcohol again!