You could also try the suggestions in the links below. Maybe Luna is holding off from doing it outside because she knows as soon as she does she has to go back in the house? I'm not saying that's the case because I don't know if you let her stay out after she does her business. She could be holding out so she can stay outside longer.
http://www.clickersolutions.com/arti...tytraining.htm http://www.clickersolutions.com/articles/2001b/bell.htm