Sounds like she has a mind of her own.
I would just try to talk to her and gently coax her to come along. When she is walking good tell her what a good girls she is.
Remmy goes through this and he is 3. There is a certain spot on our walk that he just sits his but down. It is always in the same place and once we get past it he is fine. It is in the beginning of our walk that he does this.