This has been an intermittent fault, which has increased recently. 

When I press my keyfob twice, all my doors open simultaneously. But recently, no matter how many times I press the fob, the front passenger's side door won't unlock. 
Sometimes it has unlocked using the cabin central locking button under the centre console. But lately, this hasn't worked at all.

Initially I thought this might have been a lubrication issue, so when I finally managed to get the door open by sheer luck, I sprayed the lock with some WD40 and it worked every time for about a day. Then the following day, it stopped again. 

I have checked Fuse 45 (Central Convenience System, the manual isn't very clear which one specifically controls Central Locking, so I guessed). It looks unbroken. 

It seems to only happen after the car has been in the rain. But even when the weather has dried up, it happens. 

Can anybody help, please? 

@CEG1987 - Electrics, in either front door or the bellows. Given only the LHF door appears to be affected, I'd start there.