After a few public demos and admittedly a *lot* of use of my HTC Vive Focus, unfortunately the back strap no longer stays tight. You can turn the knob and it will pull the strap in, but it immediately loosens back up.
I do not know if this is allowed, but let me try to help with that.
I also ran into this problem, and I found a couple of options (None of them are elegant enough, but if you really need to fix it yourself asap, it should do the job):
In case you do not want to disassemble rear strap, as a temporary solution you can try to find something to put in the gap to the left of the wheel, while is HMD is on your head. Anything that can lock the wheel from turning left
By red square I marked a square piece of rubber that should press the parts to each other and not let the strap to loose. This is a problematic thing. So, most likely, it has become unstuck and moved from the place and now lies somewhere where it does not do any good. Put it back to it's place (slightly to the right than it is located in my photo) and secure it in place by any means possible. A bit of good glue actually should be enough.
A small tweak that I also did, and generally I don't recommend, but you can find it usable, is to put small pieces of folded paper between the spacers in the rotary mechanism
Worked much better than I expected. What it does is give the spacers less freedom to fold, and, as a result, does not allow the wheel to turn left on its own, while does not interfere with manually twisting the wheel with your fingers in either direction
DO NOT turn the dial to relax the strap after use. Pull up the strap and remove the headset without turning the dial and remove your headset from your head, then you can loosen the strap with the dial without any cracking or slideing. Even if the headset is tight on your head, you will notice that you can remove the headset without loosening the dial.
This also happend to me, so just turn the dial to its extreams back and forth a few times and try again on your head tightnig slowly and it usually fixes itself. But to avoid this in future take of the headsset before loosening the dial.