Your going to have to stay away from the cheap stuff I'm affraid. Keep track of the formulation of the food. It's listed on the side of the bag. Brands like IAM's and Science Diet would be good to try. Or find a brand that as about the same mix. Look at the per centage of the material not just if it's there or not. You may also have to go to a nonstantard food like lamp and rice base. The better stuff may cost more but the dog will not need to eat as much. And will not produce as much waste, which is what most of the filler in the cheap food goes to. Give each new food at least a week, this will allow the dogs system to stabilize to the new food and then you should know if its going to work for you. Once you find a brand that works stay with it. Changing you brand all the time ( as in buying what ever is on sale at the time) is not good for the animal an can bring on problems also. You can't get by on junk food all the time and neither can you pet. If changing brands of food does not seem to help then you should check with a vet. The problem could also be the result of a parasite problem.