Then the pole is either blocked or it's letting in air through a hole somewhere.
Attach the pole with the machine running and listen for a change in note from the motor - if the pole is blocked it will make the same higher-pitched noise as if you'd put your hand over the air inlet - if the pole has a hole in it somewhere any change in pitch will be considerably less than when you used your hand as a cover up.
Look for blockage . look for hole as appropriate.
Peccavi
December 2012