It sounds like it could be the food. I know it has corn gluten meal in it and a lot of dogs are allergic to that.... ??
I switched Riley to the Royal Canin/Yorkie Puppy after I brought him home from the breeder.
I then switched him over to orijen puppy, which I found to be a little too rich for his system, so I switched him over to Acana.... which I Love.
Right now I am feeding him RAW in the mornings and the Acana kibble, at night. He is thriving!! It took months, but I finally found something that he and I are both happy with.
Good Luck! It can be soooooooo Frustrating.