The answer to 'why is my vacuum greasy' is that a greasy vacuum is usually caused by a buildup of dirt, debris, and grease inside the vacuum. This can happen over time as the vacuum picks up hair, dust, and other particles that contain oils and grease. A greasy vacuum can reduce suction power and make it less effective at cleaning.
Some common causes of a greasy vacuum include using the vacuum to pick up cooking oil or grease spills, vacuuming in a kitchen or other area with a lot of airborne grease, or not cleaning the vacuum regularly. Symptoms of a greasy vacuum include a sticky or oily residue on the inside of the vacuum, reduced suction power, and a musty or unpleasant odor.