DETAILED ANSWER · UPVC DOORS AND MECHANISMS
Why does a UPVC door lock when open but not closed?
Understand common symptoms across cylinders, handles and multipoint locks.
A mechanism may work with the door open because the locking points are not meeting the keeps in the frame. When closed, misalignment, a dropped door, seal pressure or worn parts may prevent engagement. Do not test by slamming or forcing the handle.
If it will not secure, describe the difference between open-door and closed-door operation when seeking help. If the door operates smoothly while open but becomes difficult only when shut, pressure or alignment may be involved. This observation is useful, but do not repeatedly lock the door in the open position as a workaround if it leaves the property insecure.
Avoid changing hinge or keep positions without checking the door system. Describe the difference and the exact point where resistance begins.