I do find bulls get complacent when they're with the same herd all the time, and perhaps especially so if they're always with the cows. That being said, perhaps a sore penis could have kept him from working for a while.
One of my best bulls was a 'one timer', he'd woo the cow for a bit, get the job done, and walk off immediately after, and his breeding rates were great, he also kept condition nicely since he went back to eating. He was an emergency replacement for a SH bull that had a bent penis, and I noticed the cows were coming back into heat (I noticed this after 1 month, so it wasn't disastrous).