There is a filter that the incoming air passes through right before it enter the bagless chamber. If that filter is clogged, wash it out with water and try again. If you already did that, remove the filter, get a coat hanger and bend it so that you can stick it down the tube right before the filter; there is likely something like a sock stuck in there that you can dislodge.
Some vacuums have another filter on the front of them. This is water washable on most newer vacuums.
Failing that, get a screwdriver and take the vacuum apart one piece at a time until you find either a clog or an air leak. Vacuums are simple, so this is an easy job. If you can't get it back together again, you just buy a new vacuum, which you were probably going to buy anyway. Worth a try.
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