Fertility Problem?

We have 4 mixed breed standard hens and 1 roo. This year and last year we had a couple go broody, but none of the eggs they sat on hatched. This also happened with our previous rooster (who was unfortunately killed by a neighbor's dog last summer). I'm not sure what the problem is. I see him doing his thing with the hens, so I have no idea what the deal is. Can hens get too old to produce fertile eggs? Our broody hens are several years old (not sure of the exact number, but they were some that we hatched out at least 3 or 4 years ago). Our other 2 hens are older than them I believe. One of them still lays eggs, but never goes broody, and I don't even think the other lays at all. I'm getting ready to introduce 2 Buff Orpington hens when they get big enough; should that help with the fertility or could there be some other factor that I'm not aware of?

Our rooster is a very pretty boy, and I'd like to hatch some chicks from him, but so far, that doesn't seem to be happening. I'd figure that with only 4 hens, the fertility rate should be practically 100%.