Most sliding patio screen doors can be unstuck with a little maintenance. First, look at the lower track. Clean it with detergent and water or even a little solvent on a rag. If the track is bent or squashed, straighten it with smooth pliers and file.
Find the rollers at the bottom of the door. Above or on the side of the rollers, you will see an adjustment screw. Use this to raise the door so it runs on the rollers rather than rubbing on the frame. You may need to lower another set of rollers located above the door, allowing the door to rise within the frame. Lubricate the rollers with silicone or a light lubricant like WD-40; don’t use other lubricants that will attract dirt.
If the door still does not operate smoothly, you may need to replace the plastic rollers in the base of the door. Or you could switch the top rollers (normally, these show little wear) with the worn bottom rollers.
By Tom Feiza, Mr. Fix-It