Blue tit's roosting

We have a new box with a camera , it's been great watching . 

The tit was in it withing an hour and after 2 days roasted over night . Then started to nest build . .but although the nest is lovely now feather lined cup she is not roosting over night and leaves about 7pm each evening - any reason why ? 

Seems odd when she has obviously decided its fir her built a nest and in and out all day not to enjoy the warmth overnight