An extractor fan draws moisture out of the air and redirects it outside, where it cannot damage your bathroom's walls and furnishings. Over time, the moving parts inside the fan may break down and cause it to lose suction or fail completely.
Check the circuit breaker box for tripped switches or blown fuses if your fan does not start when the wall switch is turned on. If this does not correct the problem, remove the extractor fan's cover and test the electrical socket inside with a voltmeter.
If you hear grinding when the extractor fan runs, the bearings inside the motor may be defective. The bearing assembly is not accessible on most models of bathroom fans, so you must replace the entire motor.
Clear out any dust or debris from the fan's vents. If this does not improve the suction, turn off the circuit breaker switch and remove the fan assembly, and then clean out the exhaust duct in the ceiling.