Hello All Its a multipoint mechanism with a barrel lock - quite common I think. So its getting on in age, and when I unlock the lock with a key then pull the handle down something sometimes catches somewhere and the door won't open. Could it be one of the "strikes" not fulling pulling out of its keep? I find myself sharply pulling the handle up and then back down again before re-trying. Sometimes need to do it a couple of times before the door will open. Is there a way to adjust/fix this ?
When you have it open are all pins moving as they should when you turn the handle? (Top, bottom and side) If they are then possibly the hinges might need adjusting. When you adjust the hinges it can move the door slightly at the handle side vertically or horizontally. It's normally an Alan key fitting accessed through a small hole in the hinge cover. Google "upvc door hinge adjustment" there's lots of videos demonstrating.