Just replaced my rear offside regulator. Similar symptoms, but I found that a bit of white plastic in the bottom of the door interior then when I pulled the inner frame off, the wires werre definitely broken - I couldn't see that until I got the inner frame off.
I bought an OEM regulator off eBay - £39 plus postage.
If your regulator really is fine but the motor spins, maybe the motor screws have come loose (doubtful) or the spines have bust in the regulator or motor.
TIP: when I tried to get the regulator off, I had to break the white plastic clip visible through the little inspection whole. When I put the new regulator in, I had to use some force to bash the new clip up onto the glass, but it worked. When I put the motor back on, I had to do very tiny turns on the motor to align it with the regulator. before tightening up the screws.