My two buff orpingtons were exactly the same. When we first got them (around 7 weeks old) they slept on the floor:
Since they were small I did not bother and let them sleep like that. Mind you that they were quite capable of roosting during the day on various setups in the coop and out in the yard:
About when they were turning 11 weeks old and still not roosting for the night, I made a point of grabbing them every night and sitting them on the roost. Once there, they stayed on it for the remainder of the night. It took about 4 or 5 days of this 'hands-on' approach for them to finally do it on their own:
I don't know if my intervention was needed but it seemed to have worked.