Most wooden screen doors sag and drag on the ground as they age. Sometimes even aluminum doors sag from poor construction or abuse. (got 14 year old boys?) The hinge side is attached to the door jamb and cannot sag, however the swinging side is prone to sagging simply because of gravity, moisture and abuse.
A simple remedy is to attach a ‘Storm Door Turnbuckle’ made to lift the outside rail of a storm door. It attaches to the bottom outside edge of the door (where it is dragging) to the hinge rail usually halfway up the hinge rail of the door.
Once screwed to the door it can be adjusted shorter, thus lifting up the swinging rail of the door.