Your horse is probably objecting to the bit lying on his tongue. Read this link about Mylar Bits. They are now approved for dressage in the US and they offer great tongue relief. http://www.actionridertack.com/blog/...-for-dressage/
I have run into several horse that hate a bit lying on their tongue. I have one now that rides entirely different (and much better) in a Mylar and opposed to any 2 or 3 piece snaffle.