I counted 38 chickens in your signature. Sounds like you free-range. That may no longer be accurate or your set-up may restrict some or several may be roosters. Only reason I mention it is that the rule of thumb is 1 nest for 4 hens, so either way, you have a lot more nesting boxes than you need. I seriously doubt the older hens are stopping your pullets from laying in the boxes.
Some people keep their hens locked up in the coop or coop and run until they lay, then let them out to roam. Mine are just as likely to lay just before they go to roost as they are to lay first thing in the morning so that would not work with mine.
I'd suggest locking the pullets in the coop or coop and run for about a week to try to teach them to lay in the nests. Keep fake eggs in the nests to show them where. Maybe if they get in the habit???? I can't think of anything else.