Cats can develop an intolerance for a food even if its one they eat every day.
Symptoms of food allergies include excessive licking, scratching at skin, ear infections etc.
You can read about food allergies here
http://www.marvistavet.com/html/body_food_allergies.html
You can compare ingredients in cat foods here
http://www.naturapet.com/display.php?d=comp-wiz&animal=Cat
You might want to try a food that has different proteins and grains in it than the one you now feed.
Hope this helps you!
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