I would eliminate all hard food, and try putting your dog on a grain free or home-made diet. Good choices are Wellness canned grain-free, Orijin canned, grain free, Evo/Inova grain free. California Naturals grain free, and others.
Make sure that your vet tested your dog's stool for
GiardiaGiardiasis. This is not a
routineRoutine sputum culture parasite check.
GiardiaGiardiasis is difficult to find in the average stool and may require several tests to find the protozoa, but it does cause blood in the stool.