Have you tried this?
From Outside the car, use the key to open (i.e. pop the little latch thing fully up).
Keep the key turned and keep the little latch thing fully up.
Use other hand to open the door.
This might work, it did for me on my Driver's Door.
Reason could be as follows:- There is a little plastic cam thing attached to the lock barrel. Its job is to push the rod down and the rod flicks the door lock open.
The plastic cam thing can actually snap after time or too much turn of the key the wrong way.
If that is your problem, you can get a new lock barrel + cam from a spares supplier for maybe £50 or you could go to a scrapyard and get a doorlock barrel and handle stuff for about £10. Unfortunately you would probably throw the scrapyard barrel away 'cos you only want the plastic cam thing and you won't get a key to fit the barrel (easily).
Hope you get sorted soon.