It is instinctual for chickens to sleep in high places. It is also instinctual for chicks to huddle, if there is no hen, they find a safe corner.
They will roost when they are ready. I don't build ladders as I want them to be able to fly up and jump down. builds good muscles. My roosts started at about 2.5', now they are 5 - 6'. They got up when they were able, and strong enough to jump down and land. We did not really encourage them to get up at all. I think if you pick them up and put them on it, they may be more likely to strain something on the landing. When raising the roosts a few of them slept on the floor for a few days before they would jump up. I imagine they tried to jump and failed, and so they slept on the floor. But eventually, they all got up.
They will get up there when they are ready. Some will stay on the floor too. every now and then we have a hen sleep on the ground. It's just how they roll. They really do know what is best.
We try to follow the mantra of letting nature take it's course. Our chickens have been raised pretty tough, but they are very healthy, and vibrant. We don't sterilize everything, they were in the dirt at 5 days old, no medicated feed, no heat lamps, outside very quickly (3 days). I believe that if you raise tough chickens, they will be much healthier and vibrant than if they are pampered. They were in a dirty environment when they were young, and have all the resistance built up naturally. If we had lost a chick, it would have better as they were weak. But, its not like we go around beating them over the head with a 2x4 either, we just require them to survive, and God has given them all the tools they need to do so very well.
Just our opinion though...