I have two 9 week old RIR hens. They insisted on sleeping on the floor in the corner. I tried for several days to put them on the perch I made from a tree post. They would panic and flap down. I decided to give up and let them continue to sleep on the ground.
We had a rain storm a couple of days ago and even though the coup/run is covered, I found them sitting on the make shift ladder. Last night when I checked on them, they were sleeping on the perch. I feel if it wasn't for the rain, they would still be sleeping on the floor.
Perhaps your chickens will eventually find sleeping on the roost is better or like me, rain will encouraging them.