passenger read door doesn't always lock / unliock when the rest of the car does

So when using the power door locks this happens ?Not when locking and unlocking it manually? If so you could have a failing door lock actuator.

No issues with manually locking the door. When I use the automatic door lock feature one of the back doors locks sometimes but not all the time. Is a failing actuator the only possibility? Is replacing the actuator something I could do? What do I need to do to get the door panel off? What do I need to know to change out the actuator? I have changed them in other cars, but this car is new to me.