I have found BNs either breed like rabbits, or not at all, with no reason I can see for either. I have had some for over 12 years. They never bred for about 5 years then suddenly there were successive broods of fry everywhere for a couple of years then they stopped again. Nothing had changed anywhere they just bred, then stopped again.
Patience is the key. They will get into it when ready. Maybe they are still settling in too.