You could try picking her up and quietly placing her on the roost with the others when it's dark. She might get the idea of where she is meant to be sleeping after a few nights of that.
Also, I know you said there was plenty of roosting space, but is there enough room for her to flap her wings and physically get herself up there without knocking the others from the perch?
The reason I ask is that I had the same problem with my girls too - Orpingtons and RIR's. They all nested on the ground in a huddle until I put in a lower roost which they could use as a step to get to the higher one.
The first night after I had installed it, I went out to check on them in the dark and they were all snuggled up together on the top roost

So sweet!
Krista.