It is very important to have a properly operating bathroom fan that is vented to the outdoors. The fan should always be run for 15-20 minutes after a shower/bath to remove excess moisture from the bathroom.
Consider changing the "manual fan switch" to either a timer switch or an automatic humidistat to ensure adequate removal of humidity.
As air and humidity are drawn through the fan, so too are dust and dirt therefore, the fan should be cleaned every six months.
For safety, before you clean the fan turn off the circuit breaker to cut the power running to it.
Remove the cover by pulling down until the cover is several inches from the ceiling. Press the spring-loaded wire clips, found on two sides of the cover together, then remove the clips from the slots in the housing.
Vacuum off the cover and then clean it in the sink with soap and water if necessary. Vacuum inside the fan housing and blades.
Reinstall the fan cover by pressing the clips on the fan together and slipping them over the slots in the fan housing. Then push the fan cover up until it seals against the ceiling.
When you’re all done, turn the breaker back on. Finally test the suction by turning the fan on and placing a tissue against it to see if the suction will hold it there