It took forever to get mine changed over to an all natural food, they are sooo picky. They now eat California Natural Chicken and Rice.
I would recommend you look and see what protein is in the food you are currently feeding and look for a higher quality food with the same protein source.
Many cats prefer a fish based food but mine are of course the strange ones who wouldnt touch them.
Try mixing the new food with the old food for awhile also and slowly adding less and less of the old food until you are adding none.
Changing to an all natural food really benefited my cats, I saw a lot less throwing up and one who had severe allergies is now fine.
However, if all else fails... chalk it up to stubborn cats and let them eat what they want!
