If teeth or bit themselves are not causeing pain, then try this:
When she moves her head up, you follow her up with your hand. Don't ever try to pull her down with a downward angled pullying hand. one sees this ALL the time. it is only going t make her hate the contact because it pulls directly on the bars. if you follow her up, keeping that straight line from elbow, through hand to bit, she gets not reward of release from the contact by raising her head. You be sure to not punish her, but to follow very evenly , neither decreasing nor increasing the contact.
She'll want to find a release from that , so she'll raise it higher, move right, left try all kinds of things , but when she lowers her head , even a tiny amount, you give a release in your hand and say "That's it! thank you".
Start with this to teach her to not raise her head as an escape mechanism. Then, as she starts lowering her head, and you release , make the release smaller and smaller, and retake up a very soft contact after she lowers her head to a normal position. This is how you work to making her accpt contact without evasion.