minibarn,
I will re-read the manual on the bobbin issue. I do know I put my bobbin in the 15-91 case the same way I do all the other machines that use class 15 bobbins and it works well.
Bobbin fit = it fits in the case, doesn't bind, and the thread rolls off it smoothly.
Not being sarcastic cos there is soooo much variation between one group of bobbins and the next. If it fits and works I use it.
Check the little peg that drives the bobbin. It could be worn.
Joe