Its probably the door actuator. I had the same problem o my 2000 S40 & i replaced with a second hand actuator. When i dismantled the old actuator i found the one of the plastic cog wheels had a wear and could not rotate the order cog wheel.

My front passenger door won't lock with the key (i dont have remote). When i lock with the key at the driver door all the doors lock except the front passenger door. But i can close/open the door from both inside and outside. The puch botton on the door will not even move when pressed. Please assist