Woodpecker nesting

I have a medium,  sub-urban garden and i have had greater spotted woodpeckers visiting for over two years. This year, there was even a juvenile greater spotted woodpecker. I have a large, coniferous fir tree and the woodpeckers will sit at the top of the tree and call before flying down into mine or other people’s gardens. There is an old plum tree with feeders on it which the woodpeckers seem to favour and they will use the feeders near the bottom of the tree and climb up the tree before flying away. I positioned a woodpecker nest box on a thin but tall tree within sight line of the top of the plum tree however there are branches and twigs covering the space between the trees in places. Recently, a female woodpecker has been visiting the nut feeder and then a male woodpecker arrived. The female woodpecker flew up to the male woodpecker so she was on the opposite side of the tree to him and then there was a brief fight midair between the two. Could this be a sign of courtship and why are the woodpeckers not using the nest box?