Chihuahua. my chi dog as studded 4 bitchs 3 yorkies and 1 chi,?

1 yorkie litter 3 pups was born early all died, 2 yorkie litter 4 pups full term 2 died. 3 yorkie litter 3 pups all alive up till now, chi litter 4 pups 1 dead others dieing, the yorkies have all had litter of 4-6 to yorkie dog, chi's first litter, why have the pups died is it the chi dogs fault because other litters was ok, there was nothing wrong with the pups that I could see all was suckling well and active and yet within minutes they died, WHY? ps he is KC reg