Citroen C3 Desire central locking issue?

Hi, having a problem with my C3 desire's central locking. The remote fob is working, although when it's battery ran out I needed to use the key. When I did this I discovered that the key only locks the drivers door and not the others. I've been told that it might be an actuator but citroen were less than helpful. Does anyone have any ideas on what it might be?