There are a couple of budget-price nightlatches that are unusually good compared with most other stuff from their manufacturer. The more expensive of the two is comparable in function to Yale’s PBS auto-deadlocking nightlatches. And unusually, it’s probably better than the PBS, which quite frequently fails fails locked and fails unlocked.
But they are fussy. If you’ve got one and you like a largely uneventful life, there are a couple checks you should make. You’ve probably got one if on the inside, there’s a large handle with a sort-of teardrop shape with a keyway in the handle (in addition to the keyway on the outside of the door).
Have a look at the bolt. Because it’s a latch lock it will have a bevelled face, which enables the lock to slam shut. Is the bevelled face straight or curved? If it’s curved then the evidence mounts that you have one of these. Is the lock loose? If it is it’s probably the cheaper of the pair – the non-British-Standard version. If it is loose, here’s the first thing to do: every couple of months put a smear (just a smear) of thick grease on this face of the bolt where it strikes its keep. And tighten the screws above and below the bolt. Just nip them up, don’t strip the thread! And don’t use WD40. (You never use WD40 on anything with moving parts! It’s a good product but its marketeers are uninformed and evil.)
Onto the second piece of advice. And the thing that prompted this post. I’ve just been to two jobs where the same mistake had been made by whoever fitted the lock. The barrel that your key goes into on the outside communicates with the lock, through the door, via a “tailpiece” or “communicating bar”. With most locks you have to shorten this when fitting the lock, unless your door is of castle thickness. And with most locks if you don’t shorten it enough, the lock simply won’t fit on the door or obviously won’t function properly. With this lock, however, if you leave the tailpiece a little too long, the lock will work, but work incorrectly.
Have a look at the bolt again. Is there a second, much thinner thingummybob next to the bolt? It has the same profile as the bolt. But if you look at the keep – the bolt’s receiver mounted on the frame – (assuming that is that the keep has been fitted correctly), you’ll see that this little gadget can’t enter the keep. In fact the keep pushes this gadget in. Let’s call this gadget the anti-slip activator. With the door open, check that you can push the main bolt in (you must be able to or the door won’t shut). Now hold the activator in and try to push the main bolt in again. This time the main bolt shouldn’t go in. The lock can’t be “slipped”. If the main bolt can still be pushed in with the activator activated, then something is wrong. You may also have suffered a fitter who left the tailpiece too long. (And you also may find that as time goes by, the lock gets more and more difficult to operate.)
This one isn’t so easily fixed. You need to be handy and mechanically adept. You have to take the lock off the door (and it isn’t trivial getting it back on again). And you need to shorten the tailpiece by only a very small amount.
If this fault is successfully fixed, and you have the more expensive, BS version, of the two, you should find one other thing starts working: when you push the activator in, a bit more of the bolt will pop out, making it even more secure.