Let's look at a few troubleshooting tips.
Check to see if there are any tears in the filter bag, this will result in for suction passing through the filter.
Check to see if all the parts are properly in place.
Check and see (depending on the model) the rubber back flow valve in the nose cap for any rips or missing completely. Suction will be greatly affected since the valve needs to be able to close to create suction.
Motor not performing at optimum performance. Make sure your unit has been fully charged. The unit will sound as if it is under performing.
The unit may need to be fully charged for 4 hours.