Try taking the stepper motor off and cleaning the gear that grabs the filament. If it's clogged it won't be able to properly feed the filament in to the extruder. There's a great video on youtube that will show you how to do it, just do a quick search and you'll see it.

Also, you should open the door and take the top off while printing PLA. You want it to cool quickly. You'd usually keep the top on and door closed while printing ABS so it has more time to cool and reduce warping.