What car do you have?
On the 78 450SEL I had until last week, the central locking would work for about half an hour after turning off the engine. The locks would remain locked after that, but the central locking and unlocking wouldn't work. I suspect there was a vacuum leak.
On my 91 300SE, I can lock and unlock the doors using the key on the driver side. I can only unlock the doors using the key on the passenger side, but I can lock the passenger door from the inside with the door open on the passenger side. Everyone tells me I shouldn't be able to do this.