[header] How to unstick a door [title] Examine the door with your eyes. [step] Look for scratches in the paint or any other noticeable damages to the door and frame. Also check for grime or any sticky buildup that may just need to be cleaned. 
Close the door and run your eyes along the gap between the door and the frame, noting places where the gap is very thin. [substeps] It is best to inspect an exterior door in a sunny or well-lit area.