Likely causes:
Diagnose by removing ALL filters and bag (Some machines have 3 filters – check the manual. Don’t do any cleaning without the filters present). Does it suck properly now? If yes, then clean or replace the filters.
Does it have a revolving brush? If so, that might have hair, fluff or fibres at either end of the brush slowing it all down. Dismantle and remove the offending material.
Does the motor noise vary for no particular reason? Possibly bearings failing (uneconomic to fix) but more likely carbon brushes worn out. Search ebay for ‘vacuum cleaner carbon brushes make model’ (eg vacuum cleaner carbon brushes Dyson DC14).
