If you switch their food all at once, and not mix it in gradually, and make the previous food less each time, that can also cause diarrhea as well. I've made that mistake before.

My cats Sophie and Honeybun both have sensitive stomachs, and they have done well on the Nutro and California Natural. I've tried the EVO, and only one of my cats was super fond of it. I kind of used it as a treat. I do free feed because I have a few that were strays when I took them in, and they still act as though they are going to starve if only leave out so much food. Sophie, for one, will gorge herself as soon as you refill a bowl that was empty and then promptly, throw it up. So, dry food stays out here all the time.