First, you will want to disconnect the hose and check the suction at the hose connection port.
Check very carefully for a blockage in the hose or the elbow and inspect the hose to ensure no tears.
Often if the hose has been regularly fully extended, then it will tear and cause suction issues. If that still seems weak, check the collection tank screen filter to ensure it is not full of hair or debris.
Remove the pre-filter and clean it well with dish soap and water.
Be sure that the filter is thoroughly dried before using it again. Any dampness getting into the motor can cause damage. Sometimes the HEPA filter is forgotten too, so if you have that, then remove that filter and inspect it.
If you have another vacuum, then vacuum the HEPA filter well and inside the HEPA filter housing.
Be sure to wipe the inside of the filter housings with a slightly damp cloth so that the filters will make a good seal. As a safety precaution, unplug it and periodically run your hand along the entire cord to ensure there are no tears or stressed points.