central locking is messing up

Hi, i have a vauxhall combo van and the central locking is playing up. when i press the fob to open up the doors, they all open up except the sliding side door. you can hear the mechanism working but the door remains locked. now the back doors have period.

Sometimes if i hold the catch up on the inside of the side door i can use the fob, press unlock
and i can open the door with the inside handle