Every single one of the chicks I brooded took until they were about 12 to 18 weeks to roost at night. They would practice roosting during the day, playing Big Chickens, but still sleep on the floor. During the day when the flock was outside, the "tween aged" chicks would go into the coop and get up onto the roosts. At night, they'd pile together on the floor. I think it has a lot to do with confidence and practice. It's gotta be hard to trust you won't fall off while you're asleep, y'know?
HEN raised chicks, however, roost really early, before momma stops raising them. When she decides she wants to roost with her flock-mates at night, the chicks gotta get up there to snuggle with her, and they do so. I've watched a hatch mother try to get chicks up on the roost at night (to shut up the anguished peeping from the floor!) for 3 nights; the first two nights she gave up and got back down on the floor with them, or even went back to the broody spot with them. The third night, the first chick managed it and the second and third chick quickly got the idea.
But without a mommy under whose wings you've been sleeping at night to encourage you, there's no real reason to get up on that roost and maneuver amongst the Big Chickens to sleep there.
Confidence and practice, yup. They need that first.
They'll learn, eventually, how to balance whilst asleep. Have patience. ;-)