I've had this problem before in a previous Alfa, it makes you appreciate central locking doesn't it!
Just take the door trim off and trace the wires to the connector block for the central locking. I had a loose connection which was held together by sticky tape! (probably a previous owners temporary fix, or a factory modification?) Obviously a loose connection so i just fixed it properly. While your there get the WD40 out, works wonders!
kind of obvious i know, just hope it's as simple as that. Never taken the door trim off my 155 though.