
Hi guest, It is a bit difficult to comment without knowing what the breed is or how old they are now.
Puppies personalities will change as they develop, it may not be anything to do with the breeder that it now seem withdrawn, it could be the litter mates are just getting a bit bolder. You don't say what type of working you want the pup for, but if I was looking for say an obedience dog, I would be looking more for a one to one interaction rather then say the boldest while with the rest of the litter.
Depending on it age, lets say it is around six weeks I would take it right away from the rest of the litter and get down on the floor and play with it. Use a ball of crinkled paper or something attached to a piece of string. Does this puppy find you more interesting then it's surroundings? This is what you want from a working dog, if it ignores you, or gets fed up with you very easily and goes off sniffing in another direction, then it is less likely it will work for you. Throw the crinkled paper, does it pick it up and at least make an effort to come towards you, or does it pick it up and run in the opposite direction?
But most of all, you have to like the puppy, if you don't it is not the one for you, walk away and look for another litter.
Hope this helps
Sandra