Blue tit in birdbox

Hi

We build a birdbox with a camera for my parents garden over 2 years ago.

For the last  2 years a single Bluetit (we assume the same one) has come in in the spring and stays the whole time.

It doesn't seem to have a partner and doesn't build a next,  is this normal?  We were hoping for a pair to breed, but just seems to chill in its own company?

Thanks for any advice in advance Blush