I have found that temperatures over 90 will affect the fertility of the rooster. As temperatures rise, I don't collect eggs to hatch when I first see a decrease during candling. When temps get cooler, fertility returns. Might try a late broody hatch if one goes broody on you. She will raise them instead of you having to do a winter brooder box.....unless of course any are nekkid!
I have had horrible hatches all year from early spring on.