Probably dumb questions but if the motor still seems to be running OK, have you checked the hose for blockages or checked that the dust canister is attached properly?
When you drop the debris can you down to empty it, you need to look up inside of the machine where the can connects to. Inside that area is a fine filter that is attached to protect the motor from the dust. It may be in a plastic frame, It may even be above a plastic or metal funnel shaped device called a cyclonic separator. You need to pull this down and clean it. This is a very normal problem with these machines as no one ever tells the home owner about this filter and when they clog the entire machine looses all of its suction power.
I suggest that you wear a dust mask when doing this. The movement of this filter will release a lot of real find dust into the air.
Take the filter outside away from the house. Place it inside your garbage can and bump it against the side of the can to release all of the fine dust partials. You may want to wash it once the bulk of the dust is removed. Then simply wait for it to dry and reinstall it and your good to go!