The screen door operation is like a bifold closet door. The door has a bottom and top pivot pin and the top track has a pivot bracket with an adjustment/ clamping screw. The issue is usually the upper pivot bracket slides in the track away from the wall letting the outer door to sag/ drop down allowing the outer guide pin/ wheel to come out of the track. The sliding bracket is caused by a loose adjustment screw or the upper threaded plate of the bracket has bent and needs to be straighten out allowing more contact with the track. After installing and aligning the door you really need to get the upper bracket adjusting screw tight and keep it tight. Getting to the screw tight can sometimes be difficult, a longer screwdriver may help you get a better grip and apply more torque to the screw.

Eddie