Since they're very new to laying they might not have connected the dots between nest box and egg laying. Since you mentioned they do climb up there to look into the nests, I think with a bit more time they'll discover the fake eggs, put 1+1 together, and start using them.
If they still don't correct themselves and you have the time to stalk them, what I did was I learned the problem bird(s) laying schedules, and when they went to their preferred spot to lay, I'd go and pick them up, stick them in a nest box, and barricade them in using my arms to cover the exit. 30-60 seconds was all they needed to calm down and start exploring the box and decide that maybe it wasn't such a bad place to be. They each started reliably using the boxes after that.
Alternatively, some folks design "doors" on their nest boxes so birds can be locked in, which basically does the same as above, but forces the bird to stay in the nest box (whereas once I see them exploring and sitting down in the nest, I leave them to it and walk away).