Hi alvaraz
many things can happen to a litter.
they are most vulnerable when they are in the process of being born, and than again the first 36-48 hours after.
many many many variables are included, and it is not a gaurantee that every pup will make it.
there are books out there on breeding that can help you to understand the vulnerabilities, and the things that can happen, it may be a good idea for those with questions to read up on the subject.
sorry for the loss this breeder suffered.