2000 Fiat Doblo central locking

Hi, I live abroad for part of the time and leave my fiat doblo parked up for up to a month at a time, I always dissconect battery, last time upon returning and connecting battery the drivers door would not unlock, now I have to unlock from passenger door and open drivers door from inside. To lock vehicle I have to start by locking passenger door then going round locking all other doors, except drivers door which appears to be permanantly locked from outside, a friend has taken the interior panel off and tells me all seems to be working ok, I have been told there could be some sort of code ???? to get it working again, any advise would be appreciated, thank you , regards Dave