Hello all! You all helped me when I needed to learn how to thread the bobbin and I'm wondering if you've ever encountered the following problem: All of a sudden (and I'm not sure exactly how sudden since a group of women now use this machine and I've not been sewing with it) the feed dogs aren't feeding fabric. The machine is threaded correctly, as is the bobbin and the machine sews but the fabric doesn't move. What could be the problem? Thanks so much for your time in advance!