from dropping out as the door is closed...the bottom point normally has a "star" shaped peg which can be screwed in or out to adjust height...but it can also be positioned in the metal base at various distances from the jamb. The top pin is set in a positionable bracket that can be moved. I like to start with the bottom and set the "star" peg first....then I loosen the fixing screw that holds the top bracket. By sliding this back and forth you can set the edge of the door plumb...the easy way is to check the gap between the door and the jamb...this gap should be the same and parallel top to bottom.

If the wheel is dropping out of the track near the middle....then the top of the door is set to far from the jamb....