Now........since your mares have no obvious fertility problems.......(whew!) And you are pretty sure of your stallion.....right?
Have you been trying to hand breed ONLY? Do your mares LIKE your stallion -- and visa versa??? (Believe it or not, this does make a difference.)
If your mares and your stallion seem to get along.......and you haven't tried pasture breeding, I would give it a try. Sometimes just letting them do their thing helps.
We have had two interesting situations over the years.......One was a mare who had a preference over which stallion she wanted to be bred to.........(and it looks like we have another similar drama this year!)
The other was a mare who came to us very high strung and nervous, with ulcer problems (which I feel was related.) The owners had tried to get her pregnant for 3 years without success.......She came to us and we weaned her off her drugs and just let her be a horse. It took another 3 years before she "chose" her stallion (we offered her two of them.) and she finally settled with the second fellow. The big difference I think was we let her do it naturally. And with her being a nervous personality at first, the pasture breeding helped. Interestingly, she is no longer high strung, but actually a pocket pet for US.
I know this sounds a little weird, but if you can watch your girls and try looking at things from their perspective.....(think like a horse?)........It may help a bit.