We use custom-designed quick adjuster hardware (we call it a "friction doubler") on many of our bags and straps. Here's what it looks like:
Left: adjuster on Everyday Messenger. Right: adjuster on SlideLITE.
First off, it might be a little confusing, but the "snap" of the handle does not lock the adjuster in place. There is no actual locking function on the adjuster. The snap in place is there in order to keep the pull handle from swinging outwards when not in use.
Secondly, the full answer to this question requires a deeper explanation of how this adjuster is designed and what makes it unique.
Quick adjustability is one of the most important features of our bags and straps. Hence, the adjusting mechanism (friction doubler) is designed to allow relatively easy adjustment. To achieve this, we added a handle. When you pull the handle, you're not just pulling the friction doubler sideways along the length of the strap, you're actually pulling it outwards a bit too. This perpendicular force is necessary to move the friction doubler. Try moving the doubler without grabbing the handle...it's much harder. Furthermore, try moving the friction doubler by holding one end of your strap and asking a friend to pull the other end. It won't budge (if it does, it's a warranty issue...please take a video and fill out our warranty form).
So, why is your strap slipping? Most likely your body or something in your environment is accidentally exerting force on the friction doubler. It could be your arm hitting the adjuster, or an object that you momentarily came into contact with, or something else along those lines. If this happens to you occasionally, practice using the quick adjust mechanism and try to train yourself to adjust your strap into place throughout the day. The adjuster handle, when pulled in-line with the strap, allows for super-fast on the fly adjustment.
If you're using the Everyday Messenger, remember that the adjuster is designed to be cinched every time you put the bag on. Cinch it tight when you're wearing the bag, and loosen it when you're accessing the bag or taking it off. If you're still having trouble with the strap slipping, then perhaps try reconfiguring the tension hook adjuster to one of the other loops (there are 3 of them). If you don't know what we're talking about, watch this video:
Still got issues with slippage?
To make sure your friction doubler is working well, do the pull test we mentioned above. Hold one end of your strap and ask a friend to pull the other end. Try to get the friction doubler to move on its own. It shouldn't budge, and we've never seen one that does in this case. If it does move, then take a video of the issue and send it to us through our warranty replacement form.