Appears the original poster isn't ever coming back, but just for the sake of adding a bit more info to the post....
...there's several diseases out there that old cows can carry without ever showing symptoms, but will cause major problems when uninfected virgin bulls become infected. The next breeding season the bulls are fine, they're just carriers, but it's okay because the cows are carriers too. Think vibrio is one of the diseases that does that. You bring in a clean virgin bull, throw him in a herd of asymptomatic carrier cows and you shouldn't be surprised if pregnancy rates are low -- and it's certainly not the bull's fault. I have no idea if that was the case here, but you shouldn't be so quick to blame the bull and the breeder.