If nothing else seems to work in regards to changing bits, make sure that you haven't "trained" her to put her tongue over the bit every time she thinks she needs a break. I have had smarter horses do this to avoid work because they know that when they do it, the driver stops and gets out of the cart to fix the bit. We have broke our big horse of this habit. He was being worked in his same "happy bit" (low port Kimberwicke) that he had been worked in for years when he started pulling this behavior. I think the first couple times it happened, he was just nervous and it was a nervous reaction. After that, he would be going along fine and put his tongue over, and we would stop to fix it. When we quit stopping and just let the reins out a little for him to fix it but not stop the gait we were in, he figured out that trick wasn't going to work anymore. No more tongue issues.
I'm not saying this is the case with every horse that has mouth issues, but it can be.
Myrna