For what is worth. I received this message from someone from The Barn that got it from Knock
What I think is happening is that people are overtightening the burrs in search for "Zero" - and binding the burrs together.
Every grinder has effectively 3 "zero points":
No1 is where the burrs are sitting "flush" perfectly in line with each other (your finger nail can feel theay are at the same level). At this point the inner can rotate inside the outer with a very slight contact. No coffee can come through. there is usually a slight but constant rub if you turn the burrs
This is a "functional zero"
No2 is where the inner burr is brought slightly inside the outer burr (you can feel a step down with your finger nail). At this point the burrs can't really turn against each other as the teeth bind together to prevent turning.
This is a genuine zero
No3 is an overtightened zero where the burrs are more than binding against each other - here they go far enough inside as to heavily lock against each other. At this overtightened point, when the dial/ burrs are then wound out, instead of the inner burr being free to lower down, it remains stuck in the teeth of the outer burr.
What starts to happen here is one of 2 things:
either the dial nut rises up slightly and the grinder is still not turning properly, or...
the silver screw which holds the burr in place begins to rotate instead, basically unscrewing. Then when you continue to turn the handle all it is doing is undscrewing that screw with no movement in the burr. Handle turns, burrs don't, coffee doesn't grind, eventually the mechanism unscrews, though the weird feeling in the hand means people don't usually go that far.
The simple, but perhaps alarming, solution when the burrs bind like this is to tap the top of the shaft against something heavy and stable or even with a hammer, and allow the inner burr to drop out so that it lowers down to match where the dial has been moved to.
The bigger solution to making this happen is not to overtighten the burrs - as they wear in against each other the dial setting for "zero" will appear to get tighter ie starting at less than 12. But there is literally no need to go that far back as the original functional zero (let alone the other 2 zeros) is the point at which coffee can't be ground fully.