Try removing the central locking fuse ( below rear seat ), then manually push down the door knob i reinsert fuse and locks should be re-syncronised. This method works for me, I have the same problem every 6 months or so, and I have not been able to resyncronise in the "usual" manner - fuse method is faster and less anoying IMHO.
I'd be curious to know exactly why this problem occurs.