Hi Mrs Wilson,
the same thing happend to my Purepower. It has nothing to do
with the height adjustment; if your machine is similar to mine,
then it has to do with the button you press down on with your
foot to release the upper body; it is probably broken. This part
can be purchased from Hoover spare parts.
Hope this answer solves your problem.
Kind regards, Mr M Wheelan.
Michael Wheelan, April 2008