I think it's a pin or something similar which prevents you from winding on until the shutter is released. If like me, you wind on almost immediately, sometimes this pin does not disengage quickly enough and jams up. Releasing pressure and trying again seemed to work every time.

My wind on lever also used to get stuck at the far end and would not always return. I just needed a very tiny amount of cleaner under it to solve both problems. I didn't put in anywhere near enough for it to find its way through to the bottom.