Well first question has he been used for breeding?
I was looking at a stallion once that the owner told me he had x number of foals, etc. The studbook said different. I called AMHA and asked - I didn't ask for names of mares, just how many stallion reports were filed and number of mares on the reports and was told. Didn't quite agree with what the seller said, so passed on him. I would think any seller would supply that information - I know I would if I said a stallion had been used for breeding.
I honestly would love to see that information on the studbook - I know there can be a number of reasons for no live foals, but a stud that has a report filed every year with x number of mares and is only siring a limited number of foals - I'd want to know why - especially if I was looking at a 'proven' stallion. I want to see the proof!
As for reasons - a low grade infection can cause significant infertility. Bad timing, low motility, infertile mares, preference for pasture over hand breeding or vice versa, not enough covers per mare, short breeding season, stress - there can be a number of reasons!
I bought a stallion that had never been used and based on his looks/pedigree we took a chance on him - beautiful foals every one! Don't know what the previous owner did not see, but glad we had him! He was funny too, first time we put him out with mares, he was like 'oh whats that', literally would have to point him at the squatting, peeing, winking mares - he was a bit timid and very do you mind with the mares. The second breeding season - he was give me another one (comically like bring me another virign - he was too funny!) And had 100% bred mares. High fetility, mares loved him and very successful boy!