On the front of the bobbin winder mechanism, there is a round part with small "teeth" around the outer edge...on the front of this is a heart shaped piece of metal...on this is a large screw and a small one. When we attempt to wind the bobbin, this large screw tightens up (on its own) and the bobbin winder stops turning...we have to loosen this screw again in order to get the bobbin winder to turn freely. I know it sounds crazy, but this is what is happening.