I found my horses absolutely hate the sweet iron bits. They gape their mouths and bull and don't seem "happy in the mouth". If that's not it I would seriously be looking at teeth and if nothing is found change dentist, then get his back and neck, in general his hoe body worked.
If that turns up nothing, change the bits, single joint egg butt is what all my horses like ( I have 5, of the 5, 3 like it, the other to, I haven't worked.) Even straight bar could be an idea. I wont ever again put forward the notion of a a french link/pea/double jointed bit, but I know I just can't stand them, and when I ride with them my horses fuss and fiddle. ;)