I had the same problem. I kept putting them in the coop until they finally did it on their own. I had one chicken that would peck at the others to keep them off the perch. One kept sleeping on the deep bedding (wood shavings). This happened for a couple of months. Now, she and the others sleep...
Okay, I have introduced a hen and 22 week old pullet to my original 24-26 week old pullets. The 22 week hides until the others are on the roost. The hen refuses to go in to the coop. I pick her up and put her in the coop and she just lays on the floor. This evening I thought "yea! she's going...