Hmm, you've had the mares checked. Have you thought to have Banner checked?
So sorry they don't seem to be getting pregnant. That's really disappointing!!
Unfortunately, I have no idea what the costs for that would be now - the last time I had that done was in 2000 or 2001 - after a groin/sheath/penis injury to our stallion. He did break their Colorado AV, that was quite entertaining! It took 6 months for him to return to normal - for the test it took some real work to get him interested at all in breeding/mounting to be collected. Though the warmth of the AV soon fixed that issue, LOL.
He seemed to become interested in mares and fertile again at the same time - that was how I determined he was ready for breeding and the first mare covered was US in foal at 21 days.