Some hand fed or tame cockatiels have trouble making the transition to being a breeder. With your male, you allowed him to treat you like a mate, rather than just a companion, so he feels a mate bond with you. Since they only bond with one mate, he has no interest in the female as long as you are still around. And there is also the possibility that they are not compatible. They choose their own mate in the wild and do not always accept the mate we choose for them.
When you decide to breed your bird, you have to give him up as a pet. The only way this pair has a chance of bonding is if you move their cage to a private location and have minimal contact with them. Only go near the cage to feed them or check on them. You can’t give him individual attention or handle him anymore. You need to remove the nest box, because a pair should not be given a nest box unless they are bonded as mates. This means they start to sit together most of the time, groom and feed each other and mate. If you see these changes, then it’s time to put the nest box back. Right now, since they aren’t bonded and he is keeping her from the nest box, her life is in danger because he could get aggressive with her and attack her. Remove the box and keep an eye on them, but from a distance. You have to sever that bond you have with him. If you want to keep him as a pet, then you need to separate them and if you still want to breed, get a different male for her.
Thank you for asking Lafeber,