Strange BlueTit behaviour ..

Hi Marioc and welcome.  So sorry you have lost some of the chicks in the way you have.  Are you sure this is ‘mum’ and not dad not quite knowing what to do?  Sometimes if the female is predated then the dads are sometimes in a panic as to quite what to do - hence why I ask.  Is mum still in the box at night do you know?  Is it possible there is something worrying her outside the box that is making her uneasy to leave?  Don’t have any answers i’m afraid - only questions.  There are other members on here with far more experience of nest behaviour than me so hoping someone can throw some light on this for you.  Please let us know what happens.  Good luck with the remaining two 
Sue