Why wont my gouldian finch baby leave the nest?

My gouldian finch baby is now aproaching its 23-24 day in life and it wont fledge. Is this normal? The parents are still feeding it in the nest box and it is looking good. I have read that they will fledge around day twenty and I was just wandering if I should be alarmed. I have seen the father calling the baby from outside the box and the baby have been seen sticking its head out of the hole but it remains in the nest.