On a good clutch, with the lever in one position the brushbar to clutch belt will be locked if the other belt is held stationary. In the other position the belt will rotate. If not, you need a new clutch. Also check the condition of the white wheel- if it's black, got flat spots etc, you need a new clutch.
if you have checked that the brushbar rotates freely and that there are no blockages at the cleaner head, then a faulty clutch is really the only possibility left.