After eliminating medical and dental causes, after double-checking saddle fit, the rider will usually come to the conclusion that the bit, for whatever reason, has become the problem.
If your horse resists your commands, it's probably trying to tell you that it's uncomfortable with the bit and the pressure it puts on it's tongue. 
Evasion or resistance is any behavior the horse employs that puts it in conflict with the action of the rider. Resistance can take different forms, many times pointing to a training issue or medical problem. However evasion specifically caused by bit resistance manifests whenever the rider asks the horse to go forward into the bit by using hands, seat or legs. 

Note! It is the hands of the rider that holds the reins and so controls the horse it's mouth. The harder the hands of the rider, the more the horse speeds up. Might say flee/ runs away from the pressure/ the pain. If this is the case, no bit will support control, comfort and trust.