Why wont my Bearded Dragons breed?

I have a male and a female bearded dragon. The male was 1 year old when i introduced the 2 1/2 year old female to him. They showed each other lots of attention and messed around. Bobbed their heads, chased each other, and got on top of each other, biting their necks, but never actually bred. They have now been together for a year and and are starting to get where they hardly show one another any attention. (They are definitely the right sex). And their environment is great. They are outside. No disturbance or problems. I tried to trick them when spring came around. They showed a little more attention, yet still never bred. I have tried everything so far and still NOTHING! (Sex is definitely positive) * Please help!!