Hi Batz,
Are you offering the right kind of nest? They like a round, open nest that looks like a bowl. It should be attached high in the cage. I would remove the food bowl and replace it with smaller cups to encourage her to look at the other nest. When you say her 3rd clutch is due, are you resting the pair in between each clutch? You can’t let her breed over and over or she will end up dying from the strain. You have to remove the nest once the chicks have left it and do not try to breed her for about 6 months – this means between every single clutch.
Thank you for asking Lafeber,
Brenda