I have an old wooden back door that leads to my garden. It has a mortice key lock on the handle, a sliding bolt about half a meter above it, and in-between these a latch lock that opens and closes freely so it offers no security but it doesn't hinder the use of the door.
This morning I went outside by opening the sliding bolt and then unlocking the handle with the key, and when I came back inside I locked the handle with the key and then slid the bolt across – nothing strange here. This evening I also used the door to go outside, opening it the same way I always do, but when I came to close it not a half an hour later the bolt wouldn't slide in and the key couldn't turn the deadbolt into the hole, as if the door was still open just enough that things wouldn't fit.
Naturally I opened the door all the way, inspected each locking device, and both were fine. I sprayed both of them with WD-40 for good measure. I inspected the door frame which was fine, and swept it out from top to bottom just in case there was a small stone that was preventing the door from fully closing. I also brushed off the outside of the door (the part that fits into the frame) and the edge around the door. I then closed the door and tried to lock it but it still wouldn't fit by the same degree as before I did anything to try and remedy it. If I lean my weight into the door, it does fit into place, and I can lock it with minor resistance. When I open the locks they are stiffer as a result of being pressed into their respective holes.
What has happened to my door? Being a back door it faces all weathers and despite high or low temperatures, wet or dry spells, it has never displayed resistance to being locked. It's as if some part of either the door or frame has suddenly moved by only a millimetre or two so the thing doesn't fit anymore. I even unscrewed and removed the useless latch lock just in case it was somehow interfering with something but it still won't close without being leaned into even though it did only this morning.
Best Answer
This type of shifting/sticking isn't uncommon with doors. Sometimes it's settling, other times it might be temperature or moisture causing swelling.
First, check the hinges and make sure that the screws are in tight. If you didn't check the door jamb on all four sides, do that.
If the door isn't closing against the stop in the door frame, check to see where it's hanging up. It could be weatherstripping or the door/frame may need sanding or planing in one or two spots.
If the door is meeting the stop on the frame evenly and the door still won't lock you may need to adjust the strike plate. You need to determine if the bolt is hitting high, low or off to the side. If you can't tell put a bit of lipstick or other colorant on the end of the bolt, shut the door and engage the bolt. You'll see a mark on the strike plate where it hit. If it's only minor interference you can use a metal file or Dremel tool to shave off enough metal from the plate to get the lock working. In most cases that will solve the problem. Otherwise you may have to shift the strike plate.