Bathroom ceiling fans aren’t just to remove bad smells but it keeps your home well-ventilated, improves air quality, helps to remove moisture from the bathroom, and prevents mold growth. So, first inspect it: The vent pipe will go up from the room all the way through to the roof. If you have an attic, go into the attic and visually inspect this exhaust pipe. Clean it out, and you should notice much better airflow in the exhaust fan.
Check for a Direct Route
If there are no obvious clogs in the exhaust pipe, check for other problems that could be affecting your airflow. Is the vent pipe made with flex duct, rather than a smooth-walled duct? Are there lots of bends and twists in the ductwork? Either one of these problems can compromise how well your exhaust fan works. Call a plumber to have the ductwork re-done, and you will notice a huge difference in how your bathroom vent works.
Clean the Fan
Over time, dust and grime will get pulled into your bathroom exhaust fan, the same way it pulls in air. Removing this dust and gunk will make your fan operate much more efficiently. First, make sure the fan is turned off. Next, get a vacuum cleaner with a suction hose attachment and clean the fan. The vacuum should draw all the dust and debris away from the blades of the fan, which will make it operate much more efficiently and effectively.
Source: https://www.doityourself.com