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