Some kids naturally run on their heels instead of the balls of their feet.
Ground shock migrates from the feet up to the head. The head bobbing that results causes blurred vision. Generally, the kids who have this type of trouble are easy to spot. They appear confused and sometimes stab at the ball like they are uncertain which one to catch. To cure this, I make a habit of having my entire team run on their toes during pregame conditioning and exercise drills.