Has the vet ruled out ALL internal parasites?

If yes, try using some GrapeFRUIT Seed Extract to kill off some of the negative things that could be in there. I suggest two treatments about nine hours apart.

about 16 hours after that use the enzymes.

I have had GREAT success with NOW Probiotic Defense. (It's a product marketed for human use, but it has helped me AND my dogs out a LOT.)