After a bit of experimenting, I found that my problem was a slipping lock rod pivot on the left hand side of the upper door frame. I used a Phillips screwdriver and moved the pivoting lever up and down to get it in correct position to lock and unlock. I then tightened the screw and it works!

If you need to cut into the door to access the locking mechanism, I made some measurements that might be of help.

Depending on how lucky you are, you might need to open only the top hole in order to adjust the pivot mechanism.

The measurements are from a spot indicated by a round dot, to its companion round dot across the hole or from the frame edge.

When measuring from the door edge to the cutouts, your door is assumed to be locked shut. So begin at the inside edge of the door frame and measure straight across. It won't be exact but it will be a ball park estimate of where you can cut to avoid as much damage to your door panel as possible.

Caution: There may be some 12 Volt wires inside your door. It would be a good idea to remove your negative battery cable before doing any cutting inside the door.

In the picture below, you can see the pivot adjusting screw, setting alone at the top left hand side. When that screw vibrates loose, the pivoting mechanism slips out of position and the door lock will no longer allow the door to open.