manicmike

Steve's right of course, and a picture would give a much clearer idea.
My first thought was that the bobbin is either:
1. Over-filled (and thus getting thread caught between bobbin and race)
2. The wrong one (seems less likely, though)

or 3. thread has become tangled in the race. You should be able to see this.

An over-full bobbin would cause this problem, but either way thread seems most likely as the cause. I'd try prying it with slightly more force with long nose pliers (and I mean slighty). If it doesn't come out then, don't yank it or something bad may happen: Removal of the race mechanism is easy enough and it's what you'll have to do to remove the bobbin (you can do it with the bobbin still in place). If thread is locking the whole mechanism in, you'll just have to go through the process of disassembling the surrounding bits until you can get at the offending cause. Have your snips ready to liberate it of course.