Rear door lock not working

Rear door lock doesn't move (or move enough) when activated from the driver's side controls.

Even if the door button is in the "up" position, it won't open from inside. In other words, it never opens from the inside.

If the door button is in the "up" position, you can however open it from outside by pulling the handle.

Is this symptomatic of an actuator problem? Can someone tell me how to fix this?