When Zig was on TOTW she had intermittent problems with loose stools. Muscle testing told me the pea protein, I seem to recall, was the issue. They are now off kibble and on a carb/veggie mix that I make, chicken, and supplements. Oh, the relief! No more issues. BUT! When I tried that on a foster dog, it nearly kilt her. She does not agree with chicken. So...she is on a beef based kibble and doing fabulously.
You can feed the "best" food in the world and if there is an ingredient in it that disagrees, it will be a problem. Some of the pricier foods have more ingredients added for antioxidants and such, and that may make them more likely to contain something that is a trigger. One of the limited ingredients foods might be helpful. Or you can keep doing what you're doing.