What happens inside the door when the locking button goes up an down.
Anything that you can see moving in the photos needs oil. With sufficient
lubrication, your key will lock/unlock the door very easily!
The important part is a plate that is easier to see in the lower photo.
This plate moves parallel to the rear edge of the door. Get some oil
behind this plate. There is a small access hole in the door of my car,
plugged with a removeable plastic plug. Using a long tube oiler, it
is theoretically possible to oil this mechanism with out disassembling
the door. I had pretty good luck.
There is also a spring on the front of the plate. Lube that too.