Exported Racetrack Video is Too Dark

All of the videos I've exported from footage shot recently on track with my VIRB XE have been very dark, regardless of what combinations of settings (resolution, frame speed, quality, etc.) I use. This is especially true on partly-cloudy days when the light coming through the windshield varies significantly as the car traverses the track. Is this a problem that anyone else is experiencing? If so, any clues on how to fix the problem will be greatly appreciated.