This is the first time I've raised chicks. Right now, at 7 weeks in, I have 19 that roost and 7 that form a ball on the floor. Last week, only 14 were roosting. So I figure they get around to it. I've read that you can go in when they are sleeping and put them on the roost.
Around 3 weeks of age I put roosts in the brooder (branches nailed to pieces of 2x4). I never really saw any sleep on them, but some would perch. These perchers were the first ones to use the coop roosts for sleeping.