Over time, microphone stands, particularly inexpensive ones, lose their ability to hold a heavy microphone at a fixed height.
What happens is that the soft bushing that is compressed when the stand clutch is tightened loses its structural integrity and its ability to clamp the upper tube in place.
It can be more than a little embarrassing for a singer to take a spin, only to return to the mic with it pointing to his waist.
You can prevent this with a small strip of gaffers tape.
Once the stand is at the right height and the clutch is tightened, put a strip of tape around the smaller upper tube, directly above the clutch.
This will hold the mic firmly at exactly the height you need with no slipping.
Be sure to remove the tape before collapsing the stand for transport. Forcing a strip of tape into the clutch mechanism can create a sticky mess.
47 N. Chatham Pkwy.
Chapel Hill, NC 27517