Cecilia,
I'm waiting on parts to finish up a FW I had to repair the bobbin hook assembly on. I said "repair" because the bobbin case base that has the finger that mounts under the needle plate was bent. How it got bent I do not know, but bent it was. And it was bent so little it was invisible to the eye. This caused the hook drive and bobbin case base to jam tight and the machine would not turn.
Since these things cost so very much to replace ... if you can find an original ... I found the bent spot and stoned it down to fit again.
The reason I'm saying this, is if your assembly is jammed, then either the screws are not in right as has been said, or perhaps it got bent.
Can you totally disassemble the bobbin hook assembly? You might have said that already and I missed it. If so use magnification and see if there are marks to indicate binding and or fit interference.
Joe