Chickens just naturally want to roost in the highest place possible, some will even roost in the rafters or outside in trees. Could be she just sees the top of the coop as the tallest safest spot to roost. If she's the leader, then others will follow eventually. If she's actually lower on the totem pole, could be that others are crowding her out and the roof is where she fits best.
If you have a small prefab coop, it may not be roomy enough for evening scuffling and adjusting around on the roosts. Some chickens just like to have their own space. Just keep putting her inside like others have suggested and it's possible she'll learn and accept that routine. You'll be able to tell over time if you need to expand your coop size.