Why are my finches breaking their eggs before having a chance to hatch?

I have 2 pair of zebra finches. They are in a large flight cage, with 2 separate houses and they paired themselves off.. One pair (lets call the Raisin's) they have had 3 eggs and the other pair (The Gatorade's) have had 2 eggs. Now both pair have broken their eggs before the eggs themselves have hatched.. What is going on? What should I do different so to have baby birds?