It happened already once a few years ago, but it sorted itself out after a few days, but this time it's been a few weeks already and it still cannot be locked. I just googled how to fix a warped door and thought I could give this a go.
If that worked consistently as an approach then no joinery firm would ever bother making sure that the stiles of a frame and panel door were made from straight grained timber, because you could always just pull it into shape with a clamp. Instead they do go to the trouble of choosing relatively straight-grained wood. The fact that it went back into position on it's own in the past says to me that the timber has some internal stresses and that the current hot, dry weather has dried it out sufficiently to cause this. Perhaps it wasn't sufficiently well kilned in the first place (i.e. it was slightly damp when the door was manufactured). It is quite possible that when the weather changes it will take on some moisture and return to it's previous shape. Wood isn't steel, it doesn't just bend, it is more like an elastic plastic with a degree of memory
Could you tell me what species it is and what the door finish is? Also have the top and bottom edges of the door been sealed/finished and does the door fit the frame side to side, or has there been any "planing-in" done to get it to fit the frame? Just trying to figure out if there is another approach which can be taken
So do you reckon I should I avoid trying altogether the clamp thing altogether? Of course I don't want to damage the door.
I'm a little wary of doing this, more because experience tells me that weighting a door down doesn't always work and that if you put too much stress into a dowelled joint or a mortise and tenon joint (the two main types of joint used on exterior frame and panel doors) you can crack the joints. With doors which have been stored leaning up against a wall (and taken on a bend) the only approach is to store it in the same way, but reversed to reverse the bend. Depending on how bad the bend is this might take a week or two in normal weather
I'm not sure what you mean by pulling the frame over?
It's more for internal doors on thinner casings or linings, but if the top or bottom corner of a door stick out from the door frame it is possible to unscrew the door frame from the wall and then twist the leg of the frame to match the twist in the door, repack and refix the leg(there are packers behind the screws) and seal it. In other words, instead of trying to bend a fairly substantial door you bend a much less substantial frame leg, which is a lot easier. You can also deal with a sticking-out door (on the lock side) by toeing-out the diagonally opposite corner on the hinge side (so a door sticking out at the top lock side corner can be pushed back in by adjusting the bottom hinge outwards - if the door has 3 hinges the middle hinge will need to be moved as well). You literally undo the hinge, pull the door out a few millimeters on the hinge side (I use a small cat's paw bar into the rebate) then refix it. This causes the diagonally opposite corner of the door to go in. Minor discrepancies on the hinge side of a door opening are generally much less noticeable. A combination of the two techniques can be useful to disguise the adjustments
The same sort of approach works on casings/linings of interior doors, although the architraves obviously do need to be removed first