I went through this with my dog. It is a food allergy. Switch brands immediately and make sure the ingredients are not the same.
We’ve tried this several times with little or no luck. I just bought some canned dog food my son’s FIL recommended but won’t try until we’re done with the salmon oil. The canned dog food is for sensitive skin and we’ll try mixing half a can in with one cup of his kibble to see if he likes it. Never run into anything he won’t eat.