You don't say how old they are, but if they're young'ens, then it takes a while for them to learn to use the roost bars for SLEEP. After all, it takes balance to do that whilst unconscious. They don't get that confidence right away.
My "Middle" batch of pullets, about 17 weeks old, have JUST started using roost bars for sleep; there are five of them and only two are roosting at night. Up until yesterday, all five crowded together in a nest box, or in two boxes next to each other. They jump on, and rest on, the roosts during the day, but haven't gotten around to sleeping up there at night yet. (Except for the first two, last night! Wahoo!)
A lot of people suggest the nest boxes be closed off at night, to prevent chickens from sleeping in them. If they're adults, I agree with that. If they're young pullets under or around 4 months old, I'd wait a bit longer just to see if they manage to get up onto the roosts at night. But that's just me - I'm kinda indulgent that way.