Why are my zebra finches NOT laying eggs?

HELP!!! I bought a nice pair of zebra finches that are a perfect match, they groom eachother and at 8 oclock every night they both settle in their nest a go to sleep. They have a large cage and a mixed diet. Ive seen the male on the females back flapping frantically, for about 2 or 3 seconds, is that them mating? She has never layed an egg and i dont know why. Can you help me please?