Ultimaker 2 ticking sound coming from feeder
Ultimately, your stepper motor is unsuccessful in driving the filament through the extruder. A couple potential reasons:


extruder temperature isn't high enough. This could either be operator error from not setting the temperature correctly or your machine is getting a false reading. Typically, for ABS/PLA, you can get away with a low extruding temperature of about 210c.
filament tension is too high. This happens if, say, your spool of filament gets kinked or wrapped around the spool spindle.
your nozzle is clogged. I've encountered where I had back flow coming out of the nozzle (filaments oozed around from the hotend at the threads). This was a result of not putting the hotend back together correctly. There was a enough hardened plastic backed up in the hotend that when I threaded the nozzle back on, it technically didn't fit back on all the way.