We have kept Zebbies for almost 10 years and never had any luck with breeding until the last 2 years. We are now on our 3rd cluch (I think that's what they are called) but they all die after leaving the nest.
The last baby was from an older mum but a new dad (the other dad died of old age so we got another male). We have at the moment 3 sets of Zebra Finches. It is so upsetting to keep losing the babies.