Have you tried taking her out on leash only and taking her to the same spot each time? Don't let her run around the yard, but keep her in a "potty area" and wait until she goes. Praise her like crazy. Show her how happy and proud of her you are and tell her each time that good girls get cheese and give her a little bite of cheese when you go back in the house and tell her again what a good girl she is.
Do I sound like a nut yet?