Let her buck (on the lunge) and then get her going. Don't make her work load suddenly huge compared to before, but make sure it IS longer than when she thinks it should stop. Also, some horses will buck if you slap them on the flank or rump, but will accept a correction without bucking if you slap them on the shoulder. Afterall, the slap comes after one nice 'ask' with your heel or ankle, and one firm 'ask', then slap, slap! brisk and firm! not tentative . Be ready for her to react. Hold one rein firm so she can't throw her head down and REALLY buck.