Although not being intimately familiar with this particular model as we don't have it over here, looking at the exploded picture >>here<<
, looks to be a switch over valve lever (31 on the diagram) which is actuated as the wand is removed?
Some ball models have a similar system. A partial blockage, broken linkage or sometimes just lubricating it often cures it on balls.
Is the brush roll spinning when the machine is reclined?
If anyone reading is more familiar with that model, do please chime in.