Adjusting DG Door

I have double glazed front and back doors fitted by Coldseal about 3 years ago.
They are the type where you pull the handle up to lock the door.
During the summer there is no problem, now that it has turned cold both doors are very stiff to lift the handle into the locking position. Presumably it has something to do with either the door or the frame shrinking slightly.
I have tried the trusty WD40 to no avail.
Does anyone know how to adjust these type of doors?
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