The problem is that she is used to doing it inside. She thinks that that is where she is supposed to do it. Now, you have to show her that that ain't right. For that, you need to persistent. You need to take her outside on at the times you think she needs to do it. After a nap, after an extended play session, and after eating. Take her outside, and don't come back in before she does it. Also, it would be best not to pick her up and take her outside, but lead her outside. That way she will learn interdependency.