Help - blue tit going bonkers

HI there

I'm.new here- we've had blue tits building a nest in bird box however tonight seems to be only one blue tit and refuses to go in bird box  tweeting and sitting on edge of whole? Going bonkers on fence and jumping on roof of box. Husband thinks wasps or something may have got in..we feel so upset for the little blue tits and not sure what to do?