It sounds like he really isn’t bonded with the new mate. It has been a while, but he may still be mourning for his old mate. They were obviously bonded since they produced chicks. Some birds will not form a pair bond, even if they get along. I would remove the nest box and give them more time to bond without the pressure of the nest box and mating. Wait 6 months and if he is still not effectively mating with her, then most likely this pair is not compatible as a breeding pair. Also, you mentioned the first mate died from egg binding. Did you remove the nest box and rest the pair for 6 months after the first clutch? It is very important to do this after every clutch, whether the eggs hatch or not. Cockatiels only breed once a year in the wild. They are not intended to have another clutch right after they just had one. A minimum 6 month rest is needed so she can rebuild her health and be ready to lay more eggs. Otherwise she is at a high risk of dying from egg binding. It is also important that she has a cuttlebone, and when you have them set up with the nest box, they need an egg food daily. This can be a commercial egg food or you can cook an egg with the shell washed crushed and cooked with the egg. You feed them this along with their other foods until the chicks are weaned. When you remove the box for the pair to rest, you can stop feeding egg food. As for the rest of the diet, they need a balanced food like pellets – not a seed mix. And they also need leafy greens, veggies and some fruit. If they do not get the best nutrition, they will not be healthy enough for breeding.
Thank you for asking Lafeber,