I think Ken's suggestion is a good one. Clearly the issue isn't related to the lock or the outside handle, so it would seem that it's being caused by the latch tongue possibly dragging too much on the inside of the striker plate doodas in the door frame. My guess is that the door has been set up to close too tightly.
I would start by standing outside the car, pressing the door inwards with my knee whilst simultaneously jiggling both the locking latch and the inner handle. If that works then you might get away with a good dose of graphite lube.
If that doesn't work, you can unscrew the latch mechanism from the inner face of the door to get the thing off, then spend a happy afternoon adjusting the catch position in the door frame to set it correctly.
