Door locks on 1999 Mercury Sable

My car has the key fob and keyless entry as well located beside the lock to get into the car, but I am having problem with the passenger door behind the driver side. The door will not unlock using any method, whether inside unlocking using the unlock all door button or when I use the fob or keyless entry. I have to manually pull up on the lock to unlock door. Any ideas what to do?

2 answers
sounds like it could be a door lock actuator you will have to take the door panel off to check it
Maybe child proof lock is on ,on that door ,just a suggestion...