I have a Whirlpool MH1150XMB1.
The vent seems like it’s not working. When I put a tissue paper underneath the vent, the tissue paper is not being pulled upward into the vent. The vent has 3 positions, HIGH, LOW, OFF. When I push the buttons, the fan above does turn on and changes per the 3 levels, but still no venting. 
The microwave and light still work in the unit. Everything else seems to be working.
I’m fairly new to this forum, so any advice, suggested parts I need, or guidance would be very appreciative!
Thanks! 