How can i fix my central locking ford ka??

How can i fix my central locking, Its a 1999, i can not lock the door (from outside) with the key, or from the inside on either the drivers door or passengers door, it locks then unlocks (if that makes sense). The only SKILLFUL way of locking the door is too ask a passer by or passenger to lock the passenger side door with the spare key whislt i lock it from my side, Skilled syncronised Ka door locking is the full tittle, lol. Please help little embarasing!!!