Entrance hole protective covers? Yes? No?

I have 3 blue tit nest boxes and 1 for Gt tits and 3 together for sparrows. One of the blue tit boxes has a protection plate and Windows for light. I used to have blue tits every year in this box up to 2017, nothing since. The other blue tit box has had no interest except for roosting and 2 years ago the third blue tit box had interest where the nest was built and then they disappeared. This year a Gt tit built in thr Gt tit box with small light Windows and left and built in the sparrow box facing south with no light and has laid 8 eggs. All of my boxes face north/ nth east except for the one sparrow one.
A few years ago I had a Gt Tit nest in the bottom of 3 hollow concrete blocks. She had 4 chicks and 3 fledged.