sensor may need to be cleaned – they sometimes accumulate paper fibers,
and need to be blown out with canned air to be able to recognize the paper tray status properly. the only other cause I am aware of would be if the paper is not loaded properly.
REGISTER or login:
Check out the printer properties. Are you trying to print from the correct paper tray?
If so, remove the paper tray(s) and try cleaning with a can of compressed air as mentioned above.