Nestling Rejection by Gouldians:

This occurs more often with first time breeding pairs as a result of cold or some other physical
stress factor, this usually occurs within the first few days of the chick hatching.

Quik gel together with Megamix given for 3 days at the critical ages of nestlings e.g hatching time and pin feather age will significantly reduce the likelihood of Gouldian finches ejecting their young from the nest.
Also please see the Pre Breeding and Breeding Program in the help section on ways to help protect this in breeding pairs.