What have you been doing to ensure fertile eggs? Have you had infertile eggs this whole time, or just since the weathers turned cold (if its cold where you are)?
I've struggled with fertility in my flock this time of the year in the past. The cold just seems to make roosters not feel very frisky. If this is something thats been going on for a while, something's up.
What breed(s) do you have? How many hens is your rooster with? How old are the hens and rooster?