It's just a fan in a box with a hole to let the air in and a hole to let the air out. The air moving through is used to cool the motor so there needs to be a filter to stop dust and grit spoiling the motor.
If you take the hose off and cover the inlet hole with your hand you should be able to feel the suction - you should also hear the motor run a bit faster - off load with no air to move. If not, then air is entering the body where it should not.
With the hose connected you should get the same effect when you cover the inlet at the brush end. If the motor runs fast without covering the end it shows that there may be a blockage in the hose - if the suction is weak it may show a split in the hose where air is getting in.
Any internal filters may sometimes be clogged which will reduce suction