Power door lock not locking or unlocking one door reliably

Whether using the key fob or pressing the button on the driver’s door, the left rear sliding door does not lock or unlock reliably. I can push the button on the door itself to lock or unlock it, though sometimes it feels like the linkage could be bound up. It wasn’t working at all, then it started working intermittently, now it’s back to not working at all.

Is it:

1) A bad power door lock actuator
2) A linkage to the power door lock actuator that needs to be cleaned and/or lubed
3) Something else