Citroen Xsara Picasso Desire - Central locking weirdness - MrStansfield

Hello!

This is my first post on here so I hope someone can help. I have a problem with the central locking on my Citroen, which has only just started happening in the last few weeks.

When I press unlock on the key all the doors unlock except the drivers side. I have to unlock it with the key. However, when I press lock again with the keyfob the passenger door doesn't lock but all the others do...!

Can anyone explain what's going on and how to fix it please?