They may just be not quite ready. A few things you can try:

1. Make the nest-boxes nice and comfy with lots of bedding.

2. Pop two or three golf balls or nest eggs into a couple of them.

3. Take them some yummy treats, go into the coop and once you have their attention, toss the treats into the nest boxes. This will convince them that the boxes are nice places and they will likely investigate them on their own after a couple of sessions. You could even leave a treat or two there for pullets that have the curiosity to check it out.

4. Be vigilant for eggs dropped just anywhere. Pullets will often do this at the beginning of coming into lay. As though they don't know what the eggs are for. Once they are laying, leave them in the coop until noon (or whenever it starts getting too hot in there.) Most hens lay in the morning and once they get the idea you can start letting them out a little earlier and a little earlier until back to normal.

Even without these measures, you may well find that the pullets figure it out for themselves. But a few clues can't hurt.