The problem is whether your dog is suffering from environmental allergies or food allergies. Buster was having issues so I had an allergy test done using a blood panel. I found out he is allergic to a lot of the food I was feeding him chicken, turkey, duck, salmon, peas and more. I found out later that allergy blood tests are not always conclusive sometimes giving a false positive. Ugh... So I am trying to determine what he is allergic to by feeding limited ingredients to him to find out what his triggers are. It is complicated. I know there is also a saliva test
NutriScan Food Sensitivity and Intolerance Test for Cats and Dogs that I am going to have done to see if I can narrow down his sensitivity. Good luck with your yorkie and their allergies.