DETAILED ANSWER · UPVC DOORS AND MECHANISMS
Why won't my UPVC door lock when closed?
Understand common symptoms across cylinders, handles and multipoint locks.
A UPVC door that will not lock can have several causes: alignment between door and frame, a handle or multipoint mechanism fault, a cylinder problem, or an obstruction. Do not keep lifting the handle or turning the key against resistance. Check only for obvious obstruction and whether the door sits evenly; a locksmith needs to inspect the actual hardware to diagnose it.
A safe check is to compare the door's visible gap around the frame and see whether it rubs at one point. Do not remove the keeps or adjust hinges by trial and error. Take note of where it catches and whether the key works with the door open.
That comparison helps a locksmith separate alignment symptoms from a mechanism fault, but it cannot confirm the cause remotely.