Bathroom light/exhaust fan fixture wiring
There is a flapper (back draft damper) in the outlet of the exhaust fan. Make sure that it opens when the fan runs and is not stuck closed by anything including a mounting screw or discharge pipe item. Make sure that all the packing material has been removed from the fan housing. You can remove the fan and motor so you can inspect the vent damper. On the roof, make sure that the discharge damper assembly will open with  any air movement.  Hope this helps.