When she stops, how far ahead of her are you? Some horses, especially young ones are very sensitive to where you walk next to them and if you get too far in front of their shoulder, they will stop. You might accidentally be sending messages to her that tell her to stop. See if walking further back than you normall do will help. And what is her cue to walk? A cluck? The word walk? Simply walking forward with a gentle tug on the lead rope? Make sure your cue is always the same. And you can always flip the lead rope behind you when you stop. Not hard because you don't want to hurt or scare her but praise the heck out of her as soon as she moves forward (in any direction). She will soon learn that forward motion is what you want and her life is easier when she works with
you. She might just want to see what happens when she stops following you.
Hope this all makes sense. Its a whole lot easier to do than it is to describe.