E.T.
My daughter had sealants come off. We probably wouldn't have noticed, but she had pretty big pieces of plastic like stuff in her mouth. She could tell her teeth felt "different" so we took her in. She grinds her teeth. I don't know if that was a factor.
My kids have my teeth. They fit very close together in the back. Both kids have had cavities in their molars. Brushing and floride alone can't get between the teeth. You don't mention flossing. If she isn't flossing, those teeth aren't getting clean in between.