hello, ok a few questions sorry
Do the parents still go into the box and feed the baby regularly? Have you noticed if it ever is near the opening, is it able to climb use it's beak, is it's feet ok? Not splayed, or some injury?
They usually are out and about by that age but you never know it might just be a little slower is it as big as the other baby? SOrry about the questions but it helps work out the problem. Also have you held the babies before and had it out side the nesting box at all?