Budgie abandoned chicks!? Help! Urgent?

The mom was taking very good care of her babies for around a week. 1 baby is 9 days old, the other baby is 7 days old, and the third baby is only a day old! There is still one more egg and I candled that egg and the baby is still a live inside and i feel the baby moving inside and hear tapping. Well the mom is starting to come out of the nest box A LOT! She comes out and her and the male keep regurgitating and when she is done, she doesn't go back inside. The babies will be calling for her and she doesn't go back in. She goes in only sometimes now. The newly hatched baby is gonna be 2 days old by tomorrow and i checked him and he isn't being fed. He should have by now. He keeps going under neath the bigger chicks so maybe the mom doesn't see him? When i move that baby to the side, he just crawls back under them. This new baby keeps chirping a lot though for food and hasn't been fed yet. It would be horrible for this baby to die. I do know that they still go in sometimes but its just that the mom isn't sitting on her babies much now and they are cuddling together for warmth and that i still don't see food in that new babies crop. So....What do i do? And how do i know for sure that the mom is definably not taking care of them. I didn't really want to hand feed them cuz I don't know how... please help!