Check for all of the following...
Dust reservoir full
Filters blocked with dirt (pre-motor and upper tank)
Airway blocked
Air entering where it should not
Motor struggling - badly worn brushes - dirty commutator
Check also for the correct operation of the brush roll assembly
If you take the hose assembly and cover the opening with your hand you should hear a distinctive rise in the pitch of the motor as it runs faster with no air to move - and slow again as you take your hand away and give it some work to do.
Good luck..., November 2014
