My friend has an 11 year old quarter horse mare. She canters on her right lead just fine, but when you ask for her left, she'll usually pick up the right, or she'll get the left lead in front, and the right lead in back. On the lunge line, if you put her in a small circle, she will get it eventually. But that's only like, 1 out of 10 tries. How do you correct this?