I'm working on this Singer 257 which has a drop-in bobbin. It's the first drop-in bobbin I've really studied, and this one acts like a ratchet...only allows motion one direction. In this video first I'll move the handwheel forward, which works fine. Then I'll move the handwheel in reverse, and you will see the bobbin case ride up on that stud in the back. At that point the handwheel won't go any further that direction.
VIDEO: https://youtu.be/jt-ffYrLh9A
Here are some photos of the stud.
So far I'm thinking maybe shim that whole piece to ride a little higher (washers) so the bobbin case won't climb over the stud when the hook moves in reverse.