You are not doing anything wrong, it's just that you are probably experiencing Murphy's Law 
Are you sure that you have a boy and a girl and not a boy, boy or girl, girl? In order to tell them apart, the boy will have a red patch on his cheek and the female will have none.
Also to encourage them to lay, you might want to get some grass clippings, or get a cotton ball and put it between the bar of the cage. So they can get their own nesting material. Also to encourage breeding. Try getting another pair, the more pair you have the better the chance of em to breed. They just seem more comfortable when you have a small colony of em.
Also provide fresh greens like celery and spinach, or any other kind a greens. Fresh fruit is also good. they would normally eat apples. Mine eats everything I put in it, though i only have two and both are males. I have no intention of breeding them plus they are 8 years old and seem to be content.
Also try offering them some bugs in a bag, you can normally get them on line or in some pet store. Giving them variety will greatly improve the chance.
They will breed throughout the year as long as the area you keep them in stays at a constant temperature all year round.
try this tips and see what happens
good luck
stinky and me