I guess I forgot to fix this (see my post above) and finally I got around to it.
Call the piston thing that goes in and out and rotates into the latch on the door, the "piston". And the "latch" is on the door. And there is the indentation in the plastic, in which the piston sits, and into which the protruding latch slides when closing the door. Now on mine, there was just above the piston (when out) and just below, on the surface of that indentation, some rub marks. These were caused by the latch being too large for the indentation. Anyway I shaved them a bit with a sharp knife, giving more clearance, and now the stickyness has disappeared. I suppose the protruding latch top and bottom could be shaved instead. Just make more clearance there. This way the latch is not being jammed with a friction fit into the indentation.
This is not a huge deal but I finally got tired of prying the door open with a knife.