Assuming all the usual checks have been done you could try something with more stability in her mouth. A mullen mouth or a hanging cheek. It sounds like she is looking for a contact but can't find one with such a 'mobile' bit.
forgot to mention all checks been done. She is fine jumping and not had a flat lesson as been working on jumping.
Tried her in a eggbutt tonight with not much change. She chews the bit, and hangs. She finds flat work hard and is always evading. Have a flat lesson booked end of the week.