English bulldog stillbirths...help!!

We just started breeding our English bulldogs this year and the first two litters out of different bitches both produced stillbirths...the first had 2 beautifully formed pups which she expelled days apart-both dead. The second had 4 beautiful pups and 1 waterpup-all dead. She expelled the first, the vet did a c-section and removed the other four dead pups-all beautiful and fully formed, but he thought they may have died a few days ago. He is at a loss. Has anyone else experienced this problem?