The Guardian Canister vacuums have a blow port under the filter. So you need to remove the filter (on the Guardian Ultra, you have to open the top of the vacuum) by lifting it up the forward. Then disconnect the hose from the normal suction end of the unit and plug it into the blower port. Now turn on the vacuum. If the hose is clogged, position the other end of the hose at the suction port.
For the Guardian canister, select the high, medium or low force for the motor. Then blow the dust out from under the furniture or use to fill an inflatable. Turn off the vacuum and replace the hose in the usual position and replace the filter when done.
If you are still unable to get the unit to blow out when connected to the exhaust port after attempting to clear the hose, separate the hose, wand and handle. Look down the hose for any object that might be creating a one-way valve. Remove the foreign object.Do the same with the other parts of the unit. Also replace the vacuum bag. If the bag is sufficiently full, the airflow will be reduced through the bag to the exhaust and cause the problem you describe.
The Electrolux Guardian Canister Vacuums are part of the Aerus line. The manuals for the vacuums are available here: http://www.aerusvacuums.com/support/manuals/index.cfm
. The Guardian Ultra is in the current model section. The Guardian is listed in the Older Model drop-down menu.
I hope this helps.
(many canister vacuums have a blower mode when you connect the hose to the exhaust (after the bag) instead of the suction side of the unit.)