Most breeders suggest a does first breeding be between 8 and 10 months of age. Does can become sterile if they aren't bred often enough (I think if they go over two years, could be wrong though). I've got a 2 yr old buck and a 1.5 yr old doe that are still going strong. My 8 month old NZW and my 1.5 yr old English Spot both just had litters. I typically breed every other month.
I must agree, heat (and cold) can affect fertility. Have you tried breeding, seperating, then putting them back together again an hour later? Rabbits are induced ovulators, which means the females don't go into heat until they are bred. Have you ever had any other litters from either of them? What breed are they?
Be sure to take the doe to the bucks cage, otherwise the either the buck will just sniff around and try to mark the does cage, or the doe will get territorial and can hurt the buck.