You probably have adequate nest boxes for fifteen hens. I have nineteen hens and six nest boxes, and they mostly all use just one favorite box.
If the hens are pullets just beginning to lay, I usually place fake eggs in the nest boxes, and that is usually enough to get the idea across. However, there's always one who insists on laying on the floor, since there're wood shavings down there, too. I try to watch for the wayward pullet to go in to lay, and if I can catch her beginning to settle into a spot on the floor, I'll move her into a nest box.
I had an older hen, just the other day, who laid an egg on the floor. It got smashed by the traffic. I cleaned it up and placed a board over that spot to discourage her, whoever it was, from laying there regularly. Usually that's enough to redirect their behavior.
If you don't have fake eggs, you can use golf balls. The color doesn't matter.
I've found that if I want the hens to lay their eggs in a certain box, all I need to do is place an egg, fake or real, in that nest. There's just something about eggs in a nest box that makes the other hens want to lay their eggs there too.