Hi Medina,
If this is a proven pair that has produced chicks before, they may need a rest or they may be getting too old to breed. If this is a young pair and this is their first clutch, it is very common for the first clutch to fail. You mentioned knowing some of the eggs are fertile so I am assuming you candled the eggs either in the nest box or removing them. This is disruptive and some pairs will abandon their eggs if they feel like they are being disturbed too much. Breeding birds need privacy, and a new breeding pair should be left alone as much as possible. While it is nice to know if eggs are good, it is best to leave the eggs alone and if they don’t hatch and the parents abandon them, you can candle them at that point. I’m not saying this is what happened, but it is something to think about next time they nest. If she fails to go back to the eggs, then wait a few days and discard them and let them try again. If they fail a second time, remove the nest box and let the pair rest for a few months.
Thank you for asking Lafeber,
Brenda