I would check door hinges for play, open the door and put pressure up and down, see if any play. Also see if the door is aligned properly.
Also it looks like the weather strips might be old and deteriorated. Try to see if you can slightly adjust the door lock and hinge position outward if it's possible at all. Also the locking mechanism could be worn where it meets the hinge. Just gotta play with it, it's all mechanical should be able to figure it out fairly quick