Does the fish react to visual stimulation from outside the tank, like reacting to your presence in front of the tank?
If it does it would exclude blindness. This is the first thing I would test for. Make sure only to give visual stimuli and not cause any vibrations or sounds. Also see if the fish reacts directly or rather indirectly to reactions of other fish in the tank.
From what I've read, krill is the food usually brought in connection with jaw locking in marine fish. Did you feed krill or foods containing krill?
Does the fish move its mouth at all in none food related situations? Does the fish yawn or stretch its mouth, move its lips?
If there is no motion at all it may be some form of localized paralysis.
Does the fish show any interest in rocks and is it trying to pick things off them? This is very typical for most large angels and not doing it at all would definitely be suspicious.
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