Bella mainly potties indoor on pads, and she never used to potty outside at all until one day, while we were walking in the park, she did it. I was so surprised and I didn't have any bags with me to pick up the nuggets, so I kind of had to kick them into the bushes
. Now I carry a bag with me every time, just in case. I think it could be a developmental thing, and your dog just hasn't realized yet that it's okay to potty outside. Maybe if you could take her on an extended leash walk at a time you're sure she has to potty, she will go out of desperation, and you can praise her profusely to let her know that it's an okay thing to do.