Great tit babies dying

I'm new here so not sure if it's the right place for any advice or help. We have great tits in camera box. 6 hatched. 3 have now died. The male never came back after feeding a bit for 1 or 2 days but the female is obviously not coping but sits on them so much instead of feeding them, very rarely she comes with food. Why is this and can we do anything to help?