Imagine the scene… your UPVC door has been awkward to lock/unlock for a few weeks. You’ve been out all day with the family, but you can’t open the door. You’re all locked out, standing in the street until you can get hold of a locksmith to let you in.
The strain caused by the door being badly adjusted has caused the gearbox to break inside the door. It’s a really common problem.
If you doors aren’t smooth to lock/unlock; either get it sorted, or be prepared for the mechanism inside to fail completely.