I would definitely look at a different food. I used to breed Bichons had horrible problems with allergies - I believe corn was a huge problem in their diet. You might want to look at
www.dogfoodanalysis.com - there you can learn about foods and also compare ingredients in food. Most yorkie owners avoid 6 star foods due to large amounts of protein & tend to feed 4 or 5 star foods.
Building a better dog really does start on the inside. I would strongly encourage you to change foods.