I'm having problems with lunging my mare at the walk. She is fine in all other gaits but at the walk she cuts?

into the circle. She does it almost every time and I can't get her to stop it. I've tried raising my hands in the air and saying yah in a stern voice to get her to stop cutting the corners. She just practically walks right into me. I've tried poking her shoulder with the lunge whip. None of these things are working. What can I do? Thanks so much for sharing your knowledge. I have let her develop a bad habit and it has become very frustrating.