Is it a larger walk-in style coop that they use at all during the day (besides the older ones when they go in to lay an egg), or a small coop that's only ever used for nesting and roosting? Asking because I'm wondering if the younger birds are used to being inside the coop at other times or if it's still unfamiliar and feels like the older girls' territory.
Is there enough space for them to roost as separate groups, and for the younger ones to get from the door to a roost without having to run a gauntlet of being pecked by the older birds?
One thing you can try to encourage chickens to roost in a particular place is to put a light there a little while before the time they'd go to roost. (Just a little torch or camping light or similar is fine - make sure it's somewhere you can easily access to take it out or switch it off without disturbing the chickens once they're roosting.) The light itself can attract them and it can also make it easier to navigate a space they're not all that familiar with, if they were struggling to find a roost in the darkness of the coop. Sometimes it can encourage the younger ones to go in earlier and snag a spot before the older ones come in, especially if one of them is positioning themselves as 'guard chicken' by the door.
But if your weather isn't that bad and you're happy that the run is secure enough to provide as much protection from predators as the coop, you could just let them continue to roost outside for now.
