What you are describing if pretty normal for brooder raised chicks that age. A broody will teach her chicks to go to the roosts at a much younger age. I've had some that do that at two weeks, though most take a little longer. Never more than 4 weeks though.
With my brooder raised chicks, I have had some take to the roosts by 5 weeks but 10 to 12 weeks is much more normal. Sometimes they are even later. And this is with the same general conditions. Usually when one figures it out, they are all roosting within a couple of days.
I think what is happening is that until one figures it out, they like to sleep in a group in an area they think is secure. Remember you are dealing with bird-brains. They think differently than you do. Once I move them to the grow-out coop where there is no light, they tend to sleep in corners, not in the middle of the floor. And these spots tend to be low. I generally leave mine in the elevated grow-out coop for about a week before I let them in their run. It is pretty normal for some or all of them to go to sleep in a group on the ground under the door that first night. It's obviously where they feel most secure.
When this happens, I just put them in the coop after dark and shut the door. Sometimes most or all go to sleep in the coop after that the next night, but I've had some hold out for more than two weeks before they caught on. That was with the majority going to sleep in the coop.
If you wish, when you put them in, keep it very dark and set some or all on the roost. One will probably catch on fairly soon, then the others should follow. But I just toss them inside and let them decide when they will roost on their own.
Good luck, but what you are seeing is pretty normal behavior. Don't worry. You are not doing anything wrong.