What size is it? How long are you watching the turt after you introduce anything other than shrimp? Are you feeding it at night? I used to have an adult alligator snapper and I could only see it eat its meals (thawed whole baby chickens) every once in a while. You can put in a bunch of live fish and crawfish and just let it take what it can over time. I'd bet that you'll notice them vanish even if you're not able to catch him in the act.