Some birds won't lay on the eggs until they usually lay the full 'set'. Also depends if this is the first time it lays eggs, if it is, its most likely still trying to figure out how to do it. Or just won't sit at all. But don't worry, at night is also when they sit the most as well they don't like human interaction when having eggs

Some birds don't start incubating until they have laid the full clutch. Give them some space.

My female zebra finch layed and egg today, and both my finches (male and female) will not sit on the egg. Also when I go near my birds they go to their egg and try to eat it. Please tell me why she isnt sitting on the egg, and will it still hatch even if my zebra finches dont sit on it?