My brooder raised chicks normally start using the roosts at night around 10 to 12 weeks of age. I’ve had some start at 5 weeks, some start later but 10 to 12 weeks is a good average. Until they move to the main roosts they either sleep together on the ground or on something low. I don’t know how low your nests and that divider are but I’d guess pretty low.
Chickens normally like to sleep on the highest thing available, but normally doesn’t mean always. It sounds like your roosts are higher than the nests so they will probably eventually migrate up there. But what to do in the meantime?
They are not going to be laying eggs anytime soon. You need the nests open when they start to lay or else you will teach them to lay somewhere other than the nests. Then you have a bad habit to break. It’s possible they could start laying at 16 weeks but that is really early. Most won’t start to lay until a month after that, if then.
I’d block off the nests so they can’t poop in them and probably so they can’t roost there if I’m visioning your layout correctly. Make them find some other place to sleep. At their age it doesn’t matter that much where it is. If they are not sleeping on the proper roosts by 14 weeks of age, I suggest you start setting them on the roosts after it is too dark for them to get down. They should catch on pretty fast. Your ultimate goal should be to have them roosting on the roosts and the nests open at 16 weeks of age. Until then, don’t stress about it. You don’t really have a serious problem.