First I want to make sure each pair is in their own cage? When breeding, you should only have one pair of birds per cage. The rare exception would be if you have a huge walk in aviary.
If by chance these birds are all together, then go ahead and remove the non-nesting pair. As to why the male is not interested, he may be too young. A male needs to be at least 18 months old, but ideally 2 years old for breeding. The female should be 2 years old at least. He may not be mature yet, or possible he is not interested in her. Not all birds will get along and not all will become good breeders. If you have another cage, I would separate them for now and let them live in cages side by side. If they finally start showing interest in each other again, then put them back together. I would not give them a nest box until they show signs of bonding. Otherwise she may lay infertile eggs and become aggressive towards him because they are not bonded.
If they still fail to bond with each other after a few months, this may be a sign that this pair does not belong together.
Thank you for asking Lafeber,