Food Issues...

Well, a few weeks ago I switched my cats back to "Chicken Soup for the Pet Lover's Soul" and since that time, my cats have been getting sick more (every few days). I don't know why this is but I'm probably going to switch back to Purina One Beyond. Does anyone have any insight as to why my kitties (in particular Sunny but both have been sick) are getting sick on this food when they've had it in the past???