problem UPVC door lock?

My problem is, when I unlock my door, I can no longer remove the key - the only way I can remove the key is to lock the door.
(Hence to lock the door from the other side, I have to unlock the door, open the door, lock the door whilst open, remopve the key, put the key back in the lock from the other side, unlock the door, close the door, lock the door, remove the key)
I have removed the barrel and tried it in the other door and it works fine (i.e. can remove key, when unlocked or locked).
What can the problem be ?

lgb2100
May 2008