Apparently crystallized PLA can build up in the nozzle of the extruder. I just finished dealing with this. I could hear the feed wheel skipping on the filament so I checked it out and did some research.
My solution was to remove the filament, get a #78 drill and a pin vise and drill it out. I did clean the shavings off the feed wheel and now everything feeds fine.
You can get the drills and pin vises at McMaster.com FYI
