Central locking problem on one door - Nissan Primastar

Hope someone can advise - central locking (using either of my 2 keys) is locking all doors on my 05 primastar except the back door.
I used the locking button on central console for the first time this week, whilst travelling alone on a motorway, and I'm wondering if this has triggered this problem, as I've never had an issue with the locking system before.

Any ideas?
Thank you