We have had the same problem with a 9 year old mare of ours. Two former foals at age 3 and 4 with no attempts to breed her until age 8. Never could get her in foal last year despite breeding her every cycle from March through November to a total of three different stallions. She cycled regularly, though at times would stay in heat for 10-12 days, then be out for 7, then come right back in. We had her cultured and biopsied with negative results. All indications were that her uterus, cervix and ovaries were all competent and non-infected. The one striking characteristic was that she is a very "messy" mare when she's in season and I frequently have to wash her hind legs because she was so soiled. Took her to a reproductive specialist who did an internal ultrasound and found she had delayed uterine clearance, i.e. she was not expelling fluid out of her uterus post-breeding. She felt that the mare was indeed conceiving, but the uterus was not a healthy environment for the conceptus, so therefore it couldn't attach and grow. She recommended that we flush the mare post-breeding to clear the fluid. When she started cycling this year we took a systematic approach:
1) First cycle - No real action taken. Bred her every other day through her cycle of 10 days like you would with a normal mare. Did not conceive.
2) Second cycle - bred her on day 5, 7, 9 and attempted to flush her on day #10. Cervix was already closed. She had ovulated several days previously, but had kept breeding past her ovulation day. Could not be flushed due to closed cervix. Did not conceive.
3) Third cycle, bred her on day 5 and 7. Flushed her on day #8 with perfect results (antibiotic flush and Oxytocin injection). Cervix was just starting to close which meant she had ovulated on the prior day. She was just confirmed 56 days in foal last week.
Just some ideas for your investigation! Good luck.