Last year I had 2 roosters in a pen with 12 hens. Darned if I could get a fertile egg. Every time one would mount a hen, the other male would come knock him off. It took me forever to figure out the problem. Finally, I removed one, and got fertile eggs! I know this is not "the norm," but it happened to me in one of my breeder pens.
Hmmm. Good job (at figuring out that problem). Thanks for the tip. Something else to watch out for.