Had the same problem with my labrador for the first 2 years of its life, the dog was ill everyday no matter what sort of food i was feeding him and he couldnt put on weight. Simple blood test from the vet solved everything. Sounds stupid but he had food intolerances, chicken beef pork and soya. Removed all these food now hes a different dog put on weight and is no longer ill. Feed my dog on natures diet wet food and half raw turkey legs now and again, not cheap but did the job. Hope this helps you.