Why isn't my ferret eating anything?

her name is skittles and all she eats is the marshal premium. i've tried every treat in the book and she doesn't show any interest in them. i've also tried a different diet because i heard that marshal is a bad food so i just tried changing it and she wont touch it! im very scared that she might starve! i try to keep her off the marshal. if u have any suggestions plz let me know! p.s.we r taking her to the vet to get her shots so maybe i can ask him