If your printer head is jammed 1 (or more) colors may print light (or not at all) even when you have sufficient ink. To unjam a head follow these steps:
Turn off and unplug the printer.
Open the casing in the same way that you would to put in a new ink or toner cartridge.
Take out the cartridge. If it is not empty then you will want to put it on paper towels or in a plastic bag.
Find the printer head nozzles. They will be small tubes or openings.
Look for a jam. This will seem like crusting or a clog.
If you see a clog get a pin or a needle.
Insert it gently into the heart of the clog and pull slowly. The clog should either pul our or crumble.
If there is still residue then you should wipe down with an baby or windex wipe.
Replace the ink, and try the print again.