The first thing to verify that the fan is able to both pull air in, and push it out. Turn the fan on with the door closed. After it has run for a couple of minutes open the door. If you notice a change in the noise of the fan the room is too tightly sealed for the fan to pull air in. At the least you will want to leave the door open a crack. Also make sure the exit point or anywhere along the vent isn't blocked.
Secondly you'll want to verify that your fan is designed to move sufficient air. Remove the fan cover and check the specification tag for its rated cfm (cubic feet per minute). Measure your bathroom, multiply
height to find the number of cubic feet (ft3) in the room. (If the room is 10' by 10' and has an 8' ceiling your volume is 800 ft3.) About eight complete exchanges per hour is the minimum you'd want for good circulation. For the example room you'd need a fan that can move 6400 ft3 in an hour.
6400 ft3 / 60 = 106.6 cfm. The fan rating must exceed this to provide adequate ventilation.
Turns and curves in the exhaust pipe slow the flow of air necessitating a larger fan. Also allow the fan to run for several minutes after the shower to ensure all excess humidity is removed.