Make sure you're standing behind her drive line (her girth area), point the whip at her hiney, and take a couple steps toward her hiney while you pop the whip at her rear and use whatever forward voice cue she knows. As soon as she goes faster, step back and say "good girl." Rinse and repeat until she gets it.
If she won't respond do NOT be afraid to smack her rear with the whip, or pop it on her butt. It should only take once or twice for her to get the message
. Always follow with a "good girl" and retreat. When you do retreat, keep your attention and energy focused on her hiney, pushing her forward. If she begins to slow, get her moving again BEFORE she breaks gait. Pop the whip and take a step or two toward her hiney. Keep your shoulders back, look UP, and "push" your energy forward toward her rear.