Have you thought of adding a probiotic powder. There are quiet a few out there. I used Yudigest for my old cocker and had problems with Harry when he changed to adult food and tried Buddy and Lola Super Tummy daily, which really helped.
Maybe look at Pooch and Mut Health and Digestion with is Hypoallergenic. The main thing is really slowly moving them over to a different type food. There are so many hypo ones now. Not mentioning names but Harrys downfall was a brand I bought in Sainsbury, ticked all the boxes but was many weeks to get back on track.
