Patience, Grasshopper!
Every single one of my chickens has taken time to roost on the roost bars/branches at night. They practice during the day a LOT for quite some time. I think they need to learn to trust their sense of balance before they will actually sleep on a roost at night.
I've had some of mine take until they were 18, 19 weeks, and then gradually, one or another of the same aged / same batch o' chicks, will get on the roost and stay there all night. Over the next week or so, the others will follow suit. I've never had any less than 12 weeks old actually roost off the floor.
Oh, wait, except for my very first GrandChick, who was actually hatched under a broody hen - her "Momma" taught her to roost at 8 weeks or so. For about six weeks, the poor hen had to sleep on the floor with the scaredy-chick. She would get on the roost and call her, call her, the chick would cry back, and for the longest time Buffy just got back down and slept on the floor with her baby.
Then Buffy gave up and just stayed on the roost. After a couple of nights, Samantha got it right and stayed up on the roost next to her.
But absolutely NONE of mine have ever just gone straight to a roost from the brooder. Not for weeks and weeks.