Most likely cause, clogged nozzels in the print head. As droplet sizes get smaller (they're down to a few microns) clogs increase from dust or impurities in the ink. Possible cures:

1. If you're using refilled or off-brand cartridges, try the printer manufacturer's ink.

2. Head cleaning, several times if necessary.

3. If your printer uses cartridges with built-in print heads (the HP and Canon multi-ink cartridges) replace the cartridge.

4. It probably wont help but you can try the head alignment function. (If your printer doesn't automatically perform an alignment when you replace a cartridge, manually do so occasionally).

5. A few high end HP printers have replaceable print heads. They are considered standard consumables and should be available where you buy ink for those printers. Canon offers replacement print heads for some of their ink jet printers. They must be ordered from Canon.

If none of the above fixes the problem, and the printer is under warranty, demand a new printer. If its not under warranty, you are the owner of a multi-function boat anchor/door stop/paper weight.

Yes, you should keep the printer clean and dusted but it wont help this problem. You can't avoid the microscopic dust that clogs print heads.