I just reassembled the entire door a few months ago. Before that I removed all dirt and grease from all the tracks, wheels, regulator and inspected everything before resinstalling.
The sprint is not broken. Out of the car both regulators would spring to the "closed" position if I used a window crank to move them. I'm guessing the driviers side spring is just a lot weaker / worn out but wanted to see what the experts thought.