Age,coop space, crowding, and pecking could all be reasons for sleeping off the roosts. When I first got mine I learned very quickly 2 3ft bars were enough for 2 7 week olds 3 almost full grown pullets and 1 hen. We added a 6 foot bar above and that almost solved the issues 3 of 6 were on the bars. But the younger 2 and 1 that took over mothering stayed on the floor. After about another 4 weeks the older pullet finally found a place on the roost but it still to the younger 2 till about 14 weeks of age to find thier own place.
There were at least a few nights I just stood there and watched to see what each did. It wasn't the younger 2s lack of trying, it really was the older ones lack of giving. I probably should have just put in yet another roosting bar, and may still have to since we got some chicks. But mine eventually fought for their right to space and everything balanced out. I would say try to find out why she sleeps on the floor first and maybe add another bar to give them all more room. I read somewhere roosts should be 2 ft per bird though they will tend to huddle to one side. That alone could solve most of the issues