I practice with different dogs every time with pretty much all my reactive dogs. It also helps generalize the behavior. You do have to modify the reaction before she takes any steps closer. Every step you take closer, the more likely she is to react to the other dog. Sort of like how in a stay you build the base behavior and make sure it is solid before you add challenges like walking away. Same concept.
What kind of reinforcement are you using? Maybe a higher reinforcement treat would help, like tripe.
I'm also trying to get a feel for what exactly you are doing. Your post makes it sound like you walk up to the dog, trying to hold her attention until she reacts? Are you constantly in motion while you practice this, or do you ever stop at a distance she can handle, do a bit of training so you have focus and move closer in baby steps?
It can also help to get the assistance of a friend who can remain stationary if strangers are getting mad for whatever reason.