Hi, uPVC door lock is sticking. When locking the door it locks ok and you can hear the mechanism lock but then when carrying on turning the key to get it out of the door it gets stuck and is very difficult to turn. Then when trying to unlock the door it gets stuck to the point where the key is almost breaking before it turns to unlock. Have tried some WD40 and when the door is open and the key inserted it locks just fine and there is no problem with sticking. Any ideas?