Old bathroom exhaust fan replacement
This looks like the same fan I put in my master bath when I built my house 22 years ago. That fan came with a "life time" warranty, so when I wanted to replace the blower fan I was advised that Broan considered lifetime to be 10 years. I read the warranty words to their representative and asked where in the statement did it say that "lifetime" was defined as 10 years. I was able to get them to send me a replacement squirrel cage fan blade but that was all they would do. Here is what I would do for your fan, remove the motor and the fan blade by taking out the whole metal plate it is mounted too and carefully clean the fan blade. Use a small screw driver and air compressor to remove the built up dirt (there should be a lot) from the cupped vanes, reach up into the air discharge area and make sure that the discharge flapper is free to swing open. Go outside to where the discharge air exits the house and make sure that there is nothing in the discharge duct that will affect the air flow. If the discharge duct is laid horizontally make sure it is laid flat. Mine is laid with-in the soft insulation and has now formed a partial trap in the vent hose. During the winter the high humidity air causes the water to condense a partially block the air flow. My solution is to run the fan for a few hours when no-one is taking a shower to remove some of the built up water in the hose. (do not turn the fan off as soon as your shower is finished). Also as @Ed Beal35 said, make sure that the bottom of the door has a gap. I have found out that the gap should be at least 1/2" and that3/4" is even better. By the way, that 3rd switch is probably for a night light bulb which mine has. Great for night time when you need just a little light. Lastly, that 80 CFM should be large enough for almost any size bathroom.