Sounds like she's stuck. Back her on the lead rope, releasing only when her hind feet take a step back. Then ask for forward, then back, then forward, etc. Do it then in the bridle. If you ask to back with the bit, and she doesn't, use the butt end of a whip or the tail of your reins to influence her, don't pull harder on her mouth. When you're riding, ask to back gently, and if she doesn't gently tap her shoulder with the end of a whip. She's just stuck.
In riding, a horse's energy is like a river- guided by the banks but not stopped by them.