Ah, in that case I bet it is the corn. Some of the most common ingredients they're sometimes allergic to include wheat, dairy, soy, corn, beef, chicken, and chicken eggs. You may find this article helpful for your problem
http://209.85.165.104/search?q=cache:qW_-ajoTiJwJ:www.peteducation.com/article.cfm%3Fcls%3D2%26cat%3D1664%26articleid%3D1 43+dog+food+allergy&hl=en&ct=clnk&cd=1&gl=us