It sounds as if you've been lenient about teaching your pullets to roost. Most will learn on their own eventually, since it's a natural instinct. But you can hasten the process. It just takes a bit of time and effort.
First you need to block the nest boxes at night. As long as you're outside doing that, go ahead and place the pullets on the perch close together, touching each other. This gives them some of the comfort and reassurance they derived from sleeping in a pile in the nest. They may hop back down, but keep repeating, placing your hand gently on their backs until they quiet down. Making a shh-shhing sound also calms them.
Uncover the nests first thing in the morning so they will be available for exploration by the almost-layers and clean and open for any who will be laying during the day. Cover again at night and place the pullets on their perch.
You should only have to go through this routine four nights at the most before the pullets are roosting on their own. But continue to cover the nests at night for a full week so they don't relapse while the roosting habit becomes entrenched.