Are you using a vent with the self closing louvers? With the stock “internal” fan, it takes very little obstruction to cause back pressure in your air flow. Even a light breeze blowing towards that side of your house can disrupt the flow.
I an external exhaust fan a couple of years ago and it has changed the level of odor and smoke released into the room. Keep in mind, for best results the fan should be the last thing the air flows through on its way to being vented. If placed anywhere else along the hose you are creating positive pressure in that pathway that will seek anywhere to release the pressure (crack, pinhole, loose connection)