If you hear the sound of the pump working for 5-15 seconds but not locking/unlocking, the cause can be a leak in the thin gum membranes in a vacuum unit located near each lock.
In my case it was the vacuum element situated in the boot door the cause of the problem. Check that at Lofty's page
Bert Rowe's-mercedes-Benz 'A'-class info. 5th Door (Boot Lid Locked, Jammed Closed), little picture under the subtitle "The boot lock".
I hope it helps
