Why is My AM Bulldogs poopie so soft?

They both have been to the vet (he said it was the food and to switch, but really couldn't advise me of "which" one to get) I tried 6 different foods and bought them at a feed store as everybody said not to feed generic brand name foods such as purina, so I put them on qulity brands like taste of the wild venison and bison, premium edge chicken and veggies, diamond (tried all, chicken, beef, venison) and 2 others I forgot the names of. PLEASE help us find the right food for these Gentle Monsters because i'm sure they are tired of having diarrea and i am tired of bleaching my backyard OH and YES i switched the new food very slowly so I KNOW it couldn't be that either. Thanx