have you tried blue buffalo fish and salmon formula. that one is expensive. It works really well fofr dogs with skin infections. Mine is a little one, and thats what I feed her.
my vet told me that dogs can get food allergies, and to feed mine what won't make her sick. He does sell dog food too. Not the kind I buy though. My dog gets itchy. her skin turns red, when she's switched to another food. Put her back on th eold stuff and it clears up.