Did the last one die when you were bringing it back home from the vet? And the vet could not tell you why any of the pups died? I'm not understanding what you mean...
I agree with ausgirl, I would have taken them all to the vet even just for a check up straight after the mother gave birth. There are wonderful vets such as the Lort Smith Animal Hospital who let you have accounts with them and pay the bill over time if you do not have enough money to pay up front.