[theBCnut]

This could be a food sensitivity to a particular protein source or a yeast infection. You can try different limited ingredient diets to see if certain proteins cause the issue or you can try low starch diets to see if not feeding the yeast in the gut helps with the issue. It could also be a case of a dog that needs a particular level of fiber in the diet. It may take a bit of detective work on your part to solve this mystery.