Bernina bobbins look like regular 15 class bobbins, but they're a little different. A Singer 15 class bobbin has 10 holes in it, while a Bernina 15 class bobbin has only 7 holes in it. The Bernina bobbin center hole is tighter, and won't go on other brand bobbin winders, but should work in the machine.
You may also have a damaged bobbin. When they get bent, they can create pretty bad problems.