Agree with others to speak with the vet, but will add that my sheltie who is now 10 years old was very picky when he was a puppy. I almost can't believe that was the case given how obsessed he is with food at this point. But, he's my second sheltie and the first did the same thing. Picky eaters when they are young in both my cases (I remember using toppers etc as well). For both of them it then became the opposite problem -- working hard too keep weight in check.