Not sure if it is something that I've done, but I ran out a bobbin this morning so I went to wind a new one. The inner wheel that I'm supposed to loosen will not budge. I've oiled it religiously since I got this machine a little while ago, and re-oiled, thinking that might be the problem, but it still won't budge. I just wound a bobbin the other day and it worked fine then. Any suggestions?