Do you have fake eggs in the nest boxes?
Even golf balls will work.
You can also purchase fake ceramic eggs at feed stores etc.
Put one in each box.
I did that with my new covered “kitty litter” boxes I use as extra nests and they started laying in them too.
I tried those kitty litter boxes after seeing yours, but girls wouldn't use them. I wanted to relieve the pressure of three wanting to lay at the same time in the same nest but they are adamant they only want the favourite one. Roo isn't a problem as she uses a different nest.
I have the girls indoors at the moment, we are being hit by a storm and are on yellow warning (red is worst) - stupid here took the tarp off the sides of the run to let in more light thinking we'd be ok as usually by now we are over the worst but stupid here forgot about global warming and how the weather has gone crazy and with winds tossing garden furniture around as though it were paper, this morning I got them straight out of bed and brought them in with us, for their safety.
I no longer have the kitty boxes as having not been used was able to return them to the store - sooooooo are using a nest box we made out of cardboard for Roo when she was ill and needed a place to lay while indoors with us.
I put the egg in and while out the room I could hear the egg bring rolled around the room (we have laminate flooring) came in to find Pedro and Jellybean playing with it

Put one of Lilah's eggs in box that has worked in getting Lilah using it but Roo kept going over to her and shoving her out of the box not liking that she's in there and I'm assuming Roo thinks it's her box so I have Roo on my lap to keep her from disturbing Lilah who is at present trying to lay

Soon as weather is better we will be attaching see through corrugated roofing to our run so we can have the sides tarp'd permanently without losing light.
Storm has been with us since weekend and so far weve been leaving our back door open so the girls can come in or go out as they please but today the storm is at its peak so we have them imprisoned till it dies down.
Lilah has laid her egg in thr box so hopefully the others will follow suit.