It kind of depends on what's causing it.
Are your roosts higher than the nest boxes? Most chickens like to roost on the highest thing they can get too. Not always all of them, but most go for high places. If the roost is not higher than the nest box, they may decide to roost in the nest box instead.
Is she just one hen? Is she broody?
How old are they? Young chicks don't start out roosting. They stay on the coop floor usually in a group until they are ready to roost. That can be anywhere from 5 weeks old until 25 weeks old. They may decide a nest box looks like a good place to roost if they are not ready to balance all night on the roosts. I've had them spend a couple of nights in the nest box while they are moving from the floor to the roosts.
If some are roosting and some are staying in the nest box, it is possible that the more dominant ones are being real brutes on the roosts. That happens sometimes when I have yooung chicks that are at the bottom of the pecking order. To keep from being beat up, they look for a safer place to spend the night.
I just don't have enough information to give you any good suggestions. Sorry.