I Breed CKCS And I Have Four Females And Have

I breed CKCS and I have four females and have a problem with females not whelping and when one did, the litter was still born with fully formed pups. They are having a full seasons, one aged 18 months being mated 4 times & vet confirmed no pregnancy. Another aged 3yrs male semen tested, her prog tested, male milked and direct insemination or the girl twice no pups. I have had a mouse plague which is now not an issue, but could that be the problem. Dogs are all energetic and healthy according to the vet, vaccinations up to date I am at witts end to the problem.
Debbie