Im not convinced on this theory but, I have a friend that live breeds and had this same problem. He done some asking around and believed he was feeding to high of a percentage of protein 20.%...so he backed of on the protein starting in the fall of the year and the following spring had many more multiples born... maybe just a coincidence but it seemed to work for him... I think there are some does that are just prone to have singles for some reason.. we have had some that had singles year after year.. very disappointing ..especially if you put money in good semen...