Electrical problem
2004 Vauxhall Meriva Front Wheel Drive Automatic 62000 miles

I've recently bought a Vauxhall Meriva but the central locking is playing up.
The back door on the passenger side will not unlock via the central locking key fob. I have to pull up the lock from the inside. I sprayed WD40 down the lock and it worked for a day then stopped again. It seems to be out of sync with the other doors as the button pops up when I lock the car. It seems secure though as when centrally locked, the door won't open even though the button is up.
Any guidance would be much appreciated.

Thank you