Start by teaching her to flex with the halter from the ground. What flexing does is unlock the poll ie the mind. If you feel she is hard mouthed and non responsive she is tense in the poll. Think of how your activity level is reduces if you tightened your neck muscles. So yes, getting her to relax in the poll can have a direct impact on her responsiveness to the bit. Google Uberstreichen exercises to help you fine tune these exercises. One way to tell if the poll is tight is to cup your hand over the bridge of the horse's nose and wiggle it side to side as tho trying to rattle his brains. If he is tense there will be no response. If relaxed his whole head will move.