I know this is an old post but did you put the cardboard in at night so the young hens can’t sleep in the nesting box and are kinda forced to roost?
I just put an extra roost in my prefab coop yesterday because Bok has decided to take up the whole original one now she is roosting again which meant the little ones spent the the night in the nesting boxes where she normally would be

Can you go out after dark and move them to the roost instead of blocking it off or will that not get them to learn what to do because I am doing it for them?