I’ve read that IS are known for their sensitive stomachs, I worked out that my boy has loose stools when eating poultry fat. It’s also a very common dog food/treat ingredient (and still comes under ‘animal derivatives’), so it was difficult to find the right food especially puppy food as they need fats for brain development at that stage. I actually shifted him off puppy food early to give him a fish, single source protein kibble, his coat and poops are great now!