Mare we had at the clinic, did all kinds of tests on her,tried hobbles, tried letting her and the stallion be across the fence from each other, tried different stallions. Did ultrasounds so they knew when she was ready to breed, did injections to time heat cycle.Everything showed normal and everything showed timing was right. Did this numerous times during breeding season. They did everything and spent lots of money on this mare, she would not let a stallion near her when she was in heat, but would be okay when not in heat. They even did AI's. Never took, she even fought during the AI's. This mare was never able to be bred for whatever reason, she would tear a stallion apart.
Maybe it just happens sometimes