The answer to 'vacuum dirty' is that a vacuum that is not cleaning effectively is likely due to a clogged filter, full dust bin, or worn brushes. This can result in poor suction, reduced cleaning performance, and even damage to the vacuum over time.
To troubleshoot a dirty vacuum, first check the filter and empty the dust bin. If the vacuum still isn't performing well, you may need to replace the filter or belts. For more serious issues like a broken motor or damaged parts, it's best to have a professional repair technician take a look.