You didnt mention what breed and age your girls are. If they are silkies, they most likely will never roost. Silkies sleep in piles. LOL If they are frizzles, you will need to put the roost very low for them, since they have trouble flying. And, if they are very young, they just dont know they are supposed to roost,
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So, if they are frizzled or silkies, put a very very low roost in place that they can hop up and down from easily, or put in a ladder or ramp to a higher roost, or just make them a nice pile of bedding in the corner. Dont let them sleep in the nest boxes.... they will poop them up.
If they are regular chickens who should be able to roost, just go in the coop each night, in the dark or with a dim flashlight, and move them up to the roost. If the light is on, they may just jump back down, but if it is dark, they will stay where you place them. After a few nights they will start to get the idea.
Note, if your roost is really really high and your coop is too narrow for them to fly up or is without any midway point to jump up and down from, they may have trouble getting up there, and worse yet, they may break an egg internally if they hit the floor too hard on the way down.
chel
PS, I tried moving some of mine when it was still light and it turned into a real circus with them jumping back down as fast as I could pick them up and place them on the roosts. I even had one use my bent over back as a step for hopping back to the floor. LOL That was in the pen that I call The Rascals.