It just about has to be either something like lepto or nutritional. I want to think nutritional. I have spent money on my fall calving cows this summer because I knew what could happen. We had a hot and dry summer at times and this puts a strain on the calves that are coming. The cow may look pretty good but they are putting less to the calf. I know I am talking cows and you are lambs but the same thing can happen. See this ewe bred and had twins but something happened midway and they didnt get what they needed. If I had a way to get after birth and a dead lamb I would send to a lab just to see. I still think nutritional and I know you said they were taken care of.