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The first place to look is the vacuum cleaner bag. When a bag is full or if it has become ripped or torn, there will be a corresponding loss of suction. Even if the bag looks intact but there is still a suction loss, change it for a new one. This way, you will know for sure that the bag is not the problem.
If the motor is going bad, the vacuum will lose suction. A bad motor may sound off key, it may smoke or it will run in fits and starts. Motors are not generally do-it-yourself jobs, and if the motor is bad, the vacuum must be replaced. On some bag vacuums, however, the motor uses a spinning plastic fan blade to achieve suction. The fan blade is attached directly to the motor axle. Dirt and debris pass through the blades and are pushed into the bag. If any of these blades have been broken, the motor may vibrate or run off key, leading to a loss of suction. Replace the fan blade if that's the case.