I knew a horse who I am sure had a significant mental block. He would only canter on the right lead no matter what you did, how correctly you asked him, how much shoulder fore you did.

I was the one riding him when he broke through it and it took asking over and over again for long enough that we were both exhausted afterwards and then immediately massive rewarding and stopping when he got it.

After that he started to be able to do it, although he'll still default to the right if not asked correctly.