My brooder raised chicks normally start roosting around 10 to 12 weeks of age. I’ve had some start at 5 weeks, I’ve had some go longer, but 10 to 12 weeks is a good average. This is where there are no older birds in with them. I’ll get to that. I’ve had broody hens take their chicks to the roosts as early as two weeks, but 4 to 5 is more normal for them. So your 11 week olds are acting a lot like mine often do. Perfectly normal.
There is no real problem with them sleeping on the floor. Silkies can’t fly so some never roost. It doesn’t hurt them at all.
Immature chickens rank lower in the pecking order than more mature chickens. You mentioned that the chicks in question are between 3 and 5 months old. That means these Jubilee Orpington are the youngest. They still hang out together instead of mixing with the other chicks. I think they are likely afraid of the others and are avoiding them. I see that all the time with mine though I have adults as well as younger chicks. The juveniles are afraid of the adults so they won’t roost with them. I put in a special “juvenile” roost, higher than the nests but lower than the main roosts and horizontally separated so the juveniles have a safe place to go that is not my nests. Right now I have 13-week-olds using the juvenile roost, adults on the main roosts, and 5-week-olds sleeping on the floor. The 13 week olds avoid the adults. The 5 week olds avoid all the older birds. I’ve had pullets use that juvenile roost until they are mature enough to force their way into the main flock, usually about the time they start to lay but sometimes a month or two later. There is no set schedule for any of this, as Donrae said, they do it when they are ready.
It sounds like you have roosts at various heights. Don’t be surprised if these Jubilee Orps continue to maintain a separate sub-flock for quite a while, hanging together during the day and sleeping separately at night until they mature enough to fully merge with the older birds. Until then they will probably avoid the older more mature birds. What you describe sounds perfectly normal to me. As long as they have enough room to avoid the older ones, and the even younger ones you have also have sufficient room to avoid, you will probably have no real problems. Mixed age flocks sort this stuff out over time.