
Mistiming the mating, using an old dog with low sperm count (though generally sperm can only fertilise the number of available eggs.
Pups may die anytime from conception to birth for any variety of reasons.
My last four litters have been small. I mated a 6 year old bitch who had not had a litter for a couple of years early in her cycle 11th to 15th day (previously whelped average litters from day 15 to 17 matings) using a young unproven dog. I suspect in her case with ehr going over her time that in fact she stood before she had ovulated and with several days of mating (to prove the male) there justr ween't enough viable sperm by the time she did. She whelped 4 and we lost one.
Teh next small litter was due to puppy losses in a bitch who had her first litter of 9, 3 years earlier, this time she carried 8, but 2 were still born, oen was laid on, and another was weak and failed to thrive so was PTS. Her next and last litter she appeared to come in season, and the dog wasn't interested in her until about day 16, so we just had one mating and brought her home. Her season was prolonged going about 10 dsays longer, and she whelped 5 days late. When she was palpated at day 28 by a vet who is very good at it, he said they didn't feel like 4 week fetuses but more like younger. So in her case again I think she was actually mated to early and there were just enough semen to get a small litter of four.
Her daughter wa very unco-operative the first time we tried mating ehr and next time we did premte tests. they idicated that she ovulated on day 12, so fertilizable ova would be from day 14 to 16. She still wouldn't stand and got mated late on day 17, more by luck than anything. She had 4 pups, so again probably just caught a few live eggs. This year we tried her again and no way woudl she accept mating :(
If the breeder was getting nothing but small litters from all their bitches then there could also be suspicion of canine herpes virus, especially if they had fading puppy syndrome in past litters. Also if the line is very inbred then after a while one side effect can be small litters.