Teeth are really quite cool, and can repair themselves to a certain extent when given enough minerals and calcium.
So I would look at the calcium and Vit D content of your formula and make sure she is getting enough.
Keep brushing her teeth, perhaps use a wet flannel to gently rub them down, rather than a rough brush. You can also use the flannel to clean her gums, and inside her cheeks, until she gets more teeth.