[Schwag]

Not having read the whole thread, I may be repeating but I would check the spring on my selector plate to make sure it is moving the cutoff lever properly.
I would also check the tension on the screw that holds it in place. Too tight won't move enough, too loose and it may not catch. I would also check the tappet spring to make sure it's tight enough to snap the nozzle back.
And maybe the cutoff lever has been damaged.

Do you have a sector gear clip installed? Sometimes if it's in backwards, it can cause nozzle/tappet problems