Zebra finches can vary between being excellent parents or being horrible parents. Sometimes it takes 2-3 clutches before they "get it" and start raising their babies.
Sometimes they just have to mature a bit. They may be physically old enough to breed, but not old enough to look after the babies. Some birds never seem to "get it".
Breeding birds can be heartbreaking at times.
All you can do is to make sure they have plenty of a variety of foods and fresh water - don't disturb them - and take the nest away after each 2 clutches to let them rest
