You might want to mix things up and take her to several places where dogs go. Gracie cannot pass up a tree or a spot where other dogs have gone. When we travel, I have a hard time getting her to go in spots on the side of the road, where dogs have NEVER gone. She will go eventually, but she waits until she almost pops holding out for a "pre-annointed" place.
I bet if you get her in the habit of going to places where other dogs have gone, eventually she will catch on.
Cathie