Is she in the nest all day long or just at night? If only at night, I've found that mostly those are the birds who are the lowest in the pecking order and are not fighting for their rights on the roost. I have a loner Partridge Rock who will roost with the others when she is laying and in the rooster's favor, but will roost separately and even out of the coop when she is not.
I've had a RIR who wanted to roost at our house and would settle into a chair on the porch..each evening I'd carry her back to the coop, where she would try to get as far away from the others as possible, even to roosting on the slanted top of the nest boxes.
She just wouldn't roost with the others and I didn't try to make her. She was clearly not accepted into the pecking order, though they didn't actually pick on her so much that one could see it. She was just a loner is all.
If your bird is doing it in the daytime, could be she is going broody. If not, you could examine her feet and legs, looking for any sores or soreness, any injury that would prevent her from getting up to the roost due to physical constraints.
If you find nothing, you could try replacing her to the roosts each night until she retrains to the roost and gets tired of being disturbed in her bed.