UPVC Door stiff to Lock

[MikeJM]

I have to force the door handle upward to get the lock to engage or disengage, as if the lock is out of line with the frame cutout where the lock engages. The lock works fine in the open position.
Does anyone know how to adjust the lock plate on the frame?

 

[MikeJM]

I`ve assumed that the problem is with the door frame plate but would be glad of any advice.