Rottweiler female had a litter of 11 pups eventually all died out of no reason?

My Rottweiler female had a litter of 11 pups eventually all died out of no reason at the second day I asked the vet and he had no answer clearly he said they died because of cold that was a bad joke because I had a heater especially for them another told it was some sort of infection I cant find the reason is that so some bitches have this problem of loosing the whole litter. I had done each n everything I could have done but I could not save them