When this happened on my Featherweight the bobbin case/shuttle was in wrong. I have a 15-91 and love it. I haven't taken the bobbin case out before, it is quite similar to the featherweight so hopefully that will help you Do you have the instruction book for your machine? Hopefully you have it, look at pages 30 - 31. It tells you how to clean the Stitch forming mechanism.
Here is a website of the manuals - just find yours
http://www.ismacs.net/singer_sewing_...e-manuals.html