Maybe they're just eating too much of it? Try just mixing a tiny bit of it with their dry food once a day. If they don't usually get the wet food, then yes, giving it to them here and there can cause tummy trouble, their system isn't used to it. But if it's tiny amounts all the time, it should be okay
My girls LOVE yogurt, the plain regular kind. I put a couple tablespoons of it on top of their dinner every night.
Just remember that when you feed anything at all, start out with a tiny amount of it and go from there. If they tolerate the tiny bit, add a pinch more next time, and so on
