"I coach a cheerleading squad. One of the girls has ADD, and she always wants to be the center of attention."
Carol Brady, Ph.D.
My guess is that, unless she's the center of attention, this girl feels personally rejected. Offer some one-on-one time to encourage her. During practice, have her focus on another cheerleader to make sure she keeps up with routines.
Ask her parents to support her progress at home with some sort of incentive program. Finally, consider the possibility that practice is scheduled at a "low point" in her medication regime—after school, for example. A different dosing schedule might help. Mention that to her parents.