What kind of dog food are you feeding? Is it a food for dogs with allergies? There a number of newer more natural foods on the market that contain meats that most dogs can tolerate.
For example: Natural Balance, Duck and Potato and Wellness. Foods that contain exotic meats such as bison and salmon are also sometimes tolerated better.
Avoid foods that can be purchased in a grocery store as they contain ingredients that can aggravate allergies.
Coincidentally tonight I read on another message board about a dog that has improved since he began taking a natural ingredient called Colostrum.