Hmm, that is a unfortunate yet, for some reason, common problem. I have no idea what to do about that one. Each Betta has it's own unique solution.
Abacus had that same problem when I first got him, he has had to eat only Betta flakes this whole time because it is the only thing he can keep down.
Bettas - Giovanni (HM), Burly (HMPK Giant).