Ford Focus 2000 will only lock when passenger door is locked 1st?

I have a ford Focus, my key fob stopped working. I have just changed the battery. During this period of time, where the key fob was not working, locking and un-locking all the doors were fine using the key.
Then one day i was only able to lock the drivers side door, by having to either lock the passenger side door from the inside 1st, then locking the drivers side using the key and it would stay locked. Or i would walk round to the passenge side and used the key and this would lock the whole car.
When the passenger side door is locked, turning the key in the drivers side door it would only lock and un-lock the drivers door and the passenger side do nothing.
If the passenger side door is not locked, and i turn the key in the divers side...the car will just lock and un-locks itself.
I have reset my key fob using the turn off x 4 times and heard two beeps. But it is doing the exact same thing when i hit my bottons, still not locking the drivers side door unless the passenger side door is closed 1st...any suggestions, PLEASE HELP!!