Zebra Finches Not Mating?

I have two zebra finches, a male and a female. The female won't allow the male to mate. They had many clutches of eggs in 2010 and had 1 brood. Then they of course pecked it. I was quite devastated. I wanted to see new life, but it wasn't meant to be. They are only 3 and I told my uncle (an expert in zebra finches), but he didn't have a clue what was wrong with them. So that's it, I do not know what's wrong with them. :(

By Billy