While printing, there is a motor that pushes the filament into the extruder head. If you have your feed rate set too high, it may try to extrude more filament than can possibly pass through the nozzle opening. This can cause the filament to back up and clog or even cause the filament to grind into the gear. You can repair this issue by firstly slowing down the print to a more manageable speed - the maximum printing speed recommended being 55 mm/sec. Once the speed is set, you will need to cut the filament back and feed a new run into the printer.
*Retraction plays a large role for extruding filament. If the retraction settings are set too high, the filament can begin to wear down and even get stuck on the feeding gears.
*Extruder temperatures are a setting that may need to be changed for every print. Too cool and the filament may not extrude out, whereas too high and the filament will just leak out of the nozzle. You want to find a place between these two extremes. Try adjusting the temperature +- 5 degrees at a time (120 max for PLA).
*A clog in the nozzle can cause filament to not come out at all or come out in thinner than normal strands. You can clear these by either pushing the filament in by hand once the head is preheated to try and push out any debris or use something harder like piano wire or a hand drill to clear out the nozzle. Please refer to this guide for steps on cleaning out a clogged nozzle.