Hi, welcome to the forum. Glad you joined.
How old are the ones sleeping outside? Are they your old established chickens or juveniles? I've had that happen when juveniles did not want to roost on the roosts with the adults.
Has anything changed? Have you added or taken away adult chickens to change the pecking order, remodeled the coop, or such?
Any known predator scares or attacks, especially in the coop?
The only way I know of getting them to roost in the coop once they've started that is to lock them in the coop at night so they have no choice until they get in the habit of sleeping in the coop again. This often takes a few repetitions.
How hard is it to find them and catch them when they are in the trees after dark? As long as you don't have lights down there where they can see, it's usually not that hard to scoop them up if you can find them and get to them. That's not always easy or safe.
Another option might be to lure them in the coop/run with food later in the day and lock them in until bedtime. You need to lure and lock all of them in, not just the miscreants. Or once you get them in leave them locked up for a week or so, not letting any out at all.
