You may want to take a good look at the bobbin case. It may have gunk, like Candace mentioned, or old oil, or it may be missing a screw or spring. (I'm guessing without seeing a pic.
) I have a blog article here:
http://www.archaicarcane.com/2013/03/spring-has-sprung/ that may help. It shows how to take the whole thing apart and clean it up and reassemble. Just be careful, there are a few small parts, and they jump.