Fog, I've had that same issue with other breeds, as well as some hatches of Marans. I use two roosts for awhile - one lower (about 18" from the floor) and one higher. Once they are roosting well on the lower one, they very quickly being using the higher one. Some folks pick their birds up & place them on the roost for a few nights. I've not done this, as I sorta want them to learn on their own. And there's really nothing "wrong" with them "roosting" on the floor. Some are just slower to catch on to roosting than others.