It sounds like the filament feed process is getting hung up somehow. The common causes already mentioned include tangled/criss-crossed filament, jammed nozzle due to shmutz or too low a temperature.

I would like to add another one: Misalignment in the filament feed path. Check that the hole in the manifold plate on top of the extruder is actually lined up with the filament feed hole in the top of the extruder carriage release lever.

You might also check that the aforementioned manifold plate is not being pulled down by its forward mounting screw such that it depresses the filament release lever. Stress there can cause the lever to release the filament. I put a spacer on the top of the fan heatsinks to give the manifold plate something to be drawn down against, and shifted the mechanism downward when tightening the two fan / heatsink bolts that hold it in place.