Pull the cover: inspect the fan to see if it is full of lint or debris, you may simply need to clean it.
While you're there: Get the model number of the fan and verify that it has the correct CFM for the size bathroom it is installed in.
If it is under rated, consider replacing the fan with one that is more powerful; some decent fans start as low as 50$ but you also need to consider the noise level that these inexpensive high CFM fans produce.
- Pull the cover verify the fan is free of debris
- turn on the fan
- ensure that the fan can spin freely
- turn the fan off - does it stop spinning immediately? (If yes replace the fan, No continue)
- turn the fan back on
- locate the exhaust vent outside and ensure the vent can open freely
- ensure that there is no debris preventing the fan from exhausting the air from the bathroom.
- do you feel any air flowing out from the exhaust? (If no, you may have a disconnect or blockage in the system, a snake camera can quickly confirm what is going on, if there is nothing blocking the vent, replace the fan)
If you have 2 fans in the same room, it would be good to verify if they are sharing the exhaust vent; if this is the case, it may be part of the problem as they both could be fighting for ventilation making them less effective.