Over time, small particles of filament may start clogging up the filament drive gear in the print head. A filament jam in the nozzle, or low quality (flaky) filament, or frequent changes of filament (pushing filament in or pulling it out) may accelerate this. So occasionally you should remove the block from over the filament drive gear, clear out any accumulated filament dust or particles, and reinstall the block.
For a video demonstration, see:
Polar 3D youtube video: WWBD – How to Unclog a Nozzle and Clear a Leaky Nozzle