Premiere Elements 7, on what computer operating system?
In Premiere Elements 7, there are various effects for lightening a dark video. After applying one of these lighting effects, have you been editing these applied effects in Edit Effects/Properties Palette after you apply them?
Adjust - Brightness and Contrast, Auto Levels....
Image Control Panel that is a default Properties Palette panel along with Motion and Opacity....for Brightness, Contrast, Hue, and Saturation.