Have you tried them on a grain-free food ? such as Orijen, but other companies are getting on the bandwagon. I notice there are a few people here whose animals haven't done well on Orijen but that must be true of virtually every food.
Have you thought of adding some probiotics to the yogurt? Or some slippery elm syrup, both good for gut issues.
If only our mealtimes were so well-organized, it's bedlam, sauve qui peut!