If your bird is sitting on the nest box for longer periods, but not laying, or if she is walking/standing with her tail down, she might be eggbound. If you can catch her, sometimes you can gently feel the egg inside. It's usually suggested that you give a warm bath, to help relax the muscles to allow the egg to pass, and you may want to put a few drops of slightly warmed olive oil in her vent to "grease" the egg for easier passage. Make sure she has enough calcium in her diet so that it isn't an issue with soft eggshells, or absent shell.
One of our birds, Twiggy the Welsummer, had a problem with being eggbound a couple of weeks ago. Eventually she passed a normal egg, encased in what appeared to be a soft, membrane only egg. We did do the olive oil to help her out, and had success. Good luck with your bird.