Eurasian nuthatch nestbox behaviour

More of a question mixed in with a fun fact. Last summer I hung up a large nestbox in my garden. Since january, the local pair of nuthatches had been showing up. It was very interesting, and I couldn’t find anything about this behaviour online. Both always came everyday around noon, calling and entering and exiting the nestbox repeatedly in some sort of display. They would also loudly peck at the inside and come out with bits of wood they removed from the interior. I loved seeing it, but since about a week ago, they haven’t showed any interest in the box at all (both are still around) and only great tits have been visiting and entering. They seemed bonded to the box, so why did they stop showing up? Did the great tits fight them off? Did they find a better box?