If the car is old it could be the hinges rusting out. Try opening the door and lift on it. If there is play this could be your problem. I had that problem on my old car (Nash's black one) on the driver side. It got so bad that the door wouldn't even close. Had to reinforce the hinge. Those doors are quite heavy and long and the stress plus corrorion takes a toll on the plate those hinges are bolted to.
-Patrick