Ok, I'll try to keep this short.
I just got my 2yr old Canadian/Percheron X gelding 3 weeks ago. When I looked at him before buying the owner had no problems getting him to take the bit. I had no issues the first 2 rides getting him to take the bit. Now however he will not take the bit. He thrashes his head, lowers his head, raises his head (everything to avoid the bit). When I get the bit in he seems fine (playing with the bit in his mouth), but he does yawn - alot. Today I noticed that he was opening his mouth really wide, stretching his neck out, and moving his bottom jaw around (almost like a yawn, but much longer,wider, and with more head movement). I do think his bit is the right size and I thought the bridle might have been tight at the brow band but I took that off and still have issues with him with the bit. He doesn't mind me putting my finger in his mouth on the left side, but on the right side he really doesn't like it (moves his head, clamps his lips and sometimes threatens to bite).