Hello Laurie,
I would do one of two things....go to another vet, get a second opinion. Or, which is what we did with one of our doxies, and that was switch to a food that had just the basics.
We have always fed our babies really good food, but one always had tummy troubles. By eliminating "stuff" from the food, we narrowed it down to that he was allergic to grain and couldn't tolerate beef.
It is worth a try. Oh, one thing we did do and that was to put him on a diet of cooked hamburger and rice for a week. That gets rid of a lot of stomach upset and lets you introduce foods back into their system.
Hope that helps.