DIAGNOSIS Most common cause is a damp or incorrectly seated filter. Secondary cause is a clog in the air path.
FIX Remove the filter. Ensure it is bone dry. Reinstall, making sure it sits flat and seals fully. Check the pre-motor filter under the dirt tank too.
IF THAT DIDN'T WORK Check hose for obstructions. Detach hose from both ends. Look through it. Check the clear cyclonic chamber at vacuum base for a packed debris wall.
HEADS UP Foam filter must be rinsed and THOROUGHLY air-dried for 24 hours before reuse. Damp filter kills suction.