Dog with chicken allergy: tummy issues, won't eat. What food to try?

It is certainly possible that the food is the problem. I couldn't say what it is about that brand that is problematic, but some dogs simply do better with certain brands than others. If she has a chicken allergy, I would make sure that whatever brand of food you choose you check extremely carefully for ingredients, and may go as far as calling up the company. Sometimes they can sneak something like "poultry by product" in at the very end of the ingredient list and it is very easy to miss. There are also prescription diets available that are very low ingredient and generally safer for pets with dietary issues. Other issues that can cause gastrointestinal issues in dogs would include inflammatory or infectious diseases, immune-mediated diseases (such as IBD), certain cancers, liver disease, and a few other less common possibilities. If she hasn't already, it might be a good idea to obtain some blood work just to see if everything is checking out OK. Ultrasound and abdominal radiographs are also possibilities if you want to explore the problem in more detail.