Hello and Welcome To BYC! Most chickens will eventually roost at night even if you don't do anything. Just be sure the perches are a good size (2x4s wide side up are popular) easy for them to get on and off of... not too high or you have steps or perches of varying heights for them to get up on. If they are sleeping in the nesting boxes do close those off.
My chicks all sleep on the ground in a heap until about 3 months. They'll graduate to a training roost about 12inches up and then when they are accepted into the flock, they move up to the big girl roost.
If it helps I found that my girls are a little scared of the dark, so they would be hanging out having a good time, then bam - nightfall. Freak out freak out freak out, and pile together for protection. Anyhoo, we fixed it by putting them to bed before it got dark. This way they get all settled in on their perches before nightfall. I have read also that all chicks not raised by a hen take a little longer to learn proper behaviors.
Yes, sometimes you need to train them. But if you can't do this, make sure the roost bars are accessible to them. You may have to make some steps for them if they are high. When you are out in the coop with them, place them on the bars and if they jump down, keep putting them back up. Eventually they will get the hang of it.
Good luck with your babies and welcome to our flock!