Do you have sound on your wingers?
If so as soon as you see the dog break down(you can tell when they put their ears up)... hit the sound first, second winger next. If she still does not succeed, may want to walk out to the fall let her find it then move up to simplify the mark. Marking takes baby steps, a 30 yard mark in cover may seem over simplified to you but it mean a great deal to the dogs confidence when it nails it every time. As stated you want the 20/80% rule you want your dog to have success on 80% of marks.