Did you get the bobbin out?
with all three of mine, I just get it with a thumb and forefinger and pinch, which will push the latch in and it should pop open. Mine are spring loaded, and they do pop open.
See the darker metal peice below the screw? that is the top of the latch part you are pinching to release. You should easily feel it if you put a finger along the back side of that knob.
This shows it open with the bobbin case removed
The case with a bobbin.
