Okay, here's one for you. Lacey would initially only go on cedar bark 'cause that is what I have around my house. But there are a lot of times that she has to walk it out so we are typically not in my yard when the urge finally strikes her. Here's the odd bit; she goes to the curb (the first few times she did this I thought she was trying to cross the road without me!) and lines her 4 paws on the cement and faces the road (her back is toward me) with her little rear end hovering over what ever material is in the strip between the sidewalk and the curb, the flood gates open. I thought it was pretty cool the first time she did it. It is one way she keeps her feet dry !
I found that she is much more tolerant of other types of mediums as she gets older.
Going on grass is not typical for her but she is motivated to do so if she smells that a lot of other dogs have been there before her. Maybe your girl is the same – try taking to a grassy area other dogs are using