You might consider checking the suction head - the whatsit on the end of the hose for an obstruction.
You might also check the suction head and the hose for places where air might be entering where it should not - the machine's case too. Splits and gaps etc.
Also run the machine and cover the air inlet - the pitch of the motor should rise as it has less air to shift - this is a quick way to check the integrity of the hose - no change in sound = bad hose.
Good luck...
January 2015