Just thinking aloud: since you have plenty of nest boxes, maybe try having one nest box without a curtain? Might be more appealing to her, who knows?
To your knowledge has she ever used the nest boxes? When you placed her inside a nest box to show it to her, how did it you do it? Just picked her up at a random time and put her in, and then let her out? What I do when trying to train a bird to use nests is:
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).