Mk1 door lock problem

Hi All. Problem. Mk1 Fabia, front passenger door lock. On trying to open all doors unlock but passenger button goes up and down and refuses to unlock. Door can be opened by pulling on inside door handle and using either open button on remote or pushing on doors open button on drivers door. Car is ok to open and lock/unlock all day then when locked and left overnight, the following morning back to square one. Any ideas please? Thanks in advance.