You have gotten so much great advice already, but I'll throw in my two cents. Use the round pen as little as possible for this issue. Most lead problems are balance and suppleness issues, and the round pen can become a crutch for her to balance on. The reason that circles, millions and millions of circles work is that they force her to carry herself - she does not have the pen or the rail of the arena to lean on. Maybe work on haunches in, too. If you watch the congress vids on youtube, you see many reiners ask for haunches in before the lead depatrure. It sets up the shoulders to be in the correct position, and lightens that all important lead leg. I was taught to ask for from the halt lead departures by using the inside leg steady on the cinch, and the outside leg behind the cinch to push the hips over to lighten the lead leg. Lift your reins slightly to pick up those shoulders, drive with your seat, and visualize the perfect departure! Oh, and when you are doing your millions of circles, be sure to keep that inside shoulder up!
This is essentially quoted word for word from my trainer today in a lesson, and I think he's a pretty awesome dude!
GL, and hope you have much success in the reining pen!