Those 15s are so easy to fix a thread jam. There are machines that jam a whole lot easier and are harder to make go again. I some time pick out a machine to use based on how easy a thread jam might be to fix... I made a thread here a while back about national clean out your bobbin day. I'll bring it back up. It tells how to get the bobbin area taken apart and put back on a 15.... Well at least some of them. It would most likely be a good time to clean everything thoroughly.