Why do reviewers say some diffusers stop smelling after a few hours?

This is one of the most common complaints in r/BuyItForLife and r/luxurycandles threads: many ultrasonic diffusers diffuse strongly at first, then the scent becomes hard to notice as your nose adapts (olfactory fatigue) or because diluted oil disperses faster. Reviewers who want a persistent throw tend to migrate to waterless nebulizing units, which keep oil undiluted.