The driver side door on my 2002 s40 won't unlock all the way. The door lock plunger opens with the key, but then when the key is returned to upright position to allow removal of the key, the door lock plunger falls back down just enough so the door never unlocks and the door handle will not open the door. From the inside, I can pull up on the lock tab plunger and it stays in the up position and does not fall down. The door hangle will open the door with no problems.

Any idea what might be wrong?