It is not unusual for puppies to experience digestive upset when changing foods and to me it seems more common when changing to "all natural" foods. The GI tract contains lots of bacteria that live in a balance, when you change foods it can cause changes in the pH of the GI tract and that leads to a shift in the bacterial population resulting in diarrhea. A new balance will eventually be achieved but that can take a week or two. You might try one of the "fresh" refrigerated or frozen foods but they can be pricey and sometimes inconvenient. I have no problem with foods like Royal Canin.