Another thought is that you might try adding privacy "curtains". I just took an old t-shirt and cut it in strips and placed it over the front of the boxes. My birds would only use one of the boxes until I did this.
As you can see, nothing fancy.
Are the boxes tall enough? You want them tall & wide enough that the girls can stand and turn around in the boxes, but not so big that multiple birds will get in them at the same time.