That was gonna be my question. Did you mix the food or just switch them over? Most times when you change a dogs food you have to mix them for about 7-14 days. I recommend a 25% new 75% old for 4 days then half and half for 4 days and then 25/75 the other way for 4 days then just the new after that. They will still go soft in the stool usually but it helps their tummies.
I feed Ollie Blue Buffalo Wilderness the Salmon blend and my boys drool over it. I do only feed ollie one cup per day sometimes 1.25 cups but not more than that. I don't want him getting chunky. He is the picture of health and I want to keep it that way. The plain pumpkin in the can does also help firm up the stools.