I would add another nest box, you'll need one down the road, then maybe as Paganrose said add some things to break up the big spaces, stumps, troughs, short tables, short roosts, or a floor nest, maybe the young ones will get more comfortable.
I would suspect your little ones are hiding from the older hen, nestboxes are safe and cozy to hang out in and many frightened young ones have done it. I would see if the older hen is picking on them too much before I blocked them out, eventually you will have to block your nests, or you can go...