Sometimes when a horse is asked to canter it's off balance and scared and scared horses will buck. When you do your lunge work with her saddled, just do walk and trot work and 3-5 rounds each at the trot then the other way. Don't do any more than 5. If you ask her to canter you are inviting her to buck. Work her 5 days doing walk trot as horses do better with daily repetition. By now she should be doing nice walk to trot transitions, trot to walk and stopping. She is now used to wearing a saddle. Don't rush this process. After the 5th day coax her into the canter (no lung whip) for a few strides then slow to trot. Don't haze or yell at her to canter but try just making kissing or clicking noises. Keep doing the transitions adding one or two canter strides before the downward transition. If she does fairly well after cantering two or three times, cut the lesson short and put her away. This will pay off the next time you work with her.