I have had some do that. They eventually went to the roost to sleep. You could move her to the roost if you would like. Sometimes they just like the feeling of protection. In the winter, they are sometimes warmer there (cause their feet are covered better than on the roost).
This isn't always the case, but check to see if she isn't getting shoved off the roost.
I had a couple of mine do that. Amidst the frenzy of finding the best spot, occasionally someone gets bumped off. Chickens don't see in the dark. If it is too dark to find their way back up, they will find the safest place on the ground they can, usually in the nest box. Shine a flashlight to illuminate the way, and if she follows it and goes back up then you know what the problem is. Of course, thats if she isn't already asleep for the night.