It seems strange to me, not so much that the AI was unsuccessful, but that the young bulls weren't successful either.
In 30 years of keeping cattle, I've never had a young bull fail! I've used many of them as young as 8 months and they've got my cows pregnant in the first month of working.
So I would be inclined, with such a high failure rate, to suspect that the fault was in your cows, - maybe a mineral deficiency or infection?