What do you do when your dog cant handle dog food wet or dry or even home made food?

I have tried everything I and my vet can think of to give to my Rat Terrier. She will be fine for about 3 months and start vomiting. She is now on Purina and has been for 6 months and recently she started vomiting again. She is 1 month pregnant tho I dont belive that is the problem. I have raised Rat terriers for years and she is the first one I have ever had that has this problem. Even the food for sensitive stomachs will not work. The unborn puppies already have been spoken for and are going to pre-screened homes. People I already know. Plz if you can help let me know. I have already tried raw food same situation.