I wouldn't correct her for not staying. If she is breaking her stay at this point in training it means you are pushing her too fast. Shorten the distance and time you are away. Start with small pivots in front of her and then back to the side. When you can do that reliably then you can pivot in front of her and take a step back. Don't start out by taking a step forward, especially with the leg that's closest to her because most dogs use the motion of your leg as a cue to start walking.
If she does get up I would just not say anything and place her back where she was and leave her for a way shorter time then you tried to.

Also one other tip is that everytime you add distance decrease the ammount of time you are away.

Hope this helps and good luck!