Does the trigger plate have it's unhooking slide and spring loaded plunger in place?
You'll have to drop the trigger plate out to see and check the slides operation. But it fits in a very narrow T slot in the plate w/the plunger and spring within the slide. One of the tiny headless screws secures it in place from the side edge of the trigger plate.
If that's not in place/missing,,or is stuck and jammed in position which can happen very easily, then the hook will not let go of the cocking arm.
Another thing is that on a parts gun, the hook may be just a little too big as far as the tip of the 'hook' is concerned. Not being able to release itself from the cocking arm even though the slide is pushing it back.
Just some thoughts..
