Loose stools! So common in puppies, to the extent most pups are to a greater or lesser extent a bit loose. There are several reasons why this could be so.
Firstly there is the stress factor. We take a young puppy away from everything it knows, It’s mother and littermates, the place which it called home ever since it was born, even the only people it has ever known. It’s easy to imagine how this could upset it’s system.
Secondly comes food and drink. You know when you go on holiday how even the water tasted different!
Thirdly, also concerning food. Puppies grow so fast, and need large quantities of food to sustain that growth. But they only have little stomachs. Next time you feed him, look at him and imagine how small his stomach must be in order to fit in that body, then look at the food and ask yourself, “Where on earth does he put it all?” The answer is that a large part of the food passes through him so quickly that it is only partly digested, pushed through by the next meal! This is probably the biggest reason for loose stools and is perfectly normal.
Fourthly we come to the less good things. There are a number of parasites and organisms which could cause problems, Worms, Campylobacter, Salmonella and the like. But these are not that common. Most breeders have a wormimg program so it’s rare these days that a pup from a responsible breeder will have a worm burden, but for all that, it’s possible.
If you are really worried then your vet is the person to talk to. They can test a stool sample and determine if there really is a problem or not. But the fact that some samples are firmer would lead me to guess that you probably do not have a problem. I’ve had pups remain a little loose for the first nearly 6 months simply due to the quantity of food required. The stools just gradually improve until you realise that they are now nicely formed.