I would try different mouthpices, if you find the horse does not go as well in a bit with a metal mouth piece and better in a rubber one (or another none metal mouthpiece) then you will get the idea.
I rember once when one of mine had a bad tooth, I was showing at Royal Cornwall. Every time the bit touched his tooth, He threw his head up, hollowed hios back and BUCKED!!!!!
I hadnt noticed b4 about his tooth as he was eating fine