A common problem, the locking knob on my microphone stand no longer tightens. Taking it apart, a small metal 'collar' seems to have had the thread partially stripped. It's a tiny part but I've no idea if these are standard and can be replaced.
It looks like this, for scale the knob is 1" diameter:
This is the product link: http://www.tonedeafmusic.co.uk/products/mc01b-black-mic-stand-(large--folding-with-boom).html
You could try
Alternately you could try lodging a small piece of plastic cable tie in "nut" ,simulating a Nyloc Nut,which should allow locking knob thread to grip.
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Can't think of anything else that will take its place. It's basically a semi-captive nut, that won't turn once the rod is inside the tube. A couple of ideas - use a bigger bolt, and tap out the meat to match it. Weld an appropriate nut to the outside, Both of which I've done in the past. As a semi-permanent you could put a pipe clip on the rod, to hold it up. Done that too! For less than £20 for a new stand, I doubt the manufacturers will replace the ailing part. If it always stays at a particular height, gaffer tape will do the job. Makes you look like a real muso, too...!
