Often, puppies have loose stools because you're feeding them too much. I would run it past your vet, first, of course, before I started reducing the amount of food I gave my puppy, but I might reduce by 1/4th cup first to see if that improved things. If you notice stool firming up, there's your problem.
And, sometimes the food you're feeding them just doesn't agree with them. That's not uncommon, either.
And, sometimes the food you're feeding them just doesn't agree with them. That's not uncommon, either.