This is usually caused by your sliding door not being level. This normally leaves a gap at either the top or the bottom where the door meets the panel. The sliding door can be leveled by adjusting the sliding door wheels. You do this by putting a screwdriver in the hole on the edge of your sliding door (when the door is open). Turn the adjustable screw and this will lower or height the sliding door on that side. You can adjust this on the back and the front of the sliding door at the top and at the bottom until the sliding door is level. By doing this you will not be changing or damaging your sliding door as these wheels are designed to be adjusted.