Whenever you take a shower, there is a large amount of steam released in the air. Depending on how hot you like your showers, this could be very significant. All that moisture can do damage to your bathroom, such as encourage mold growth or peel off your wallpaper. This is why it is important to have a functioning, correctly installed bathroom fan.
One of the simplest ways you can make your fan more efficient is to clean up any dust buildup in the fan itself, since this can hamper operations, lessen airflow and even pose a fire hazard.
The ductwork from your bathroom should vent outside of your home. If it vents anywhere else, such as in your attic or a crawl space, you might have a mold problem starting there instead. The condensation from the exhaust will also lessen the effectiveness of any insulation material where it collects.
The ducts should also be kept clean from debris and dust buildup, but to resolve this, there might be a need to consult a professional duct cleaner.