Cooktop exhaust vent cover...
I am sure I am the last to know this, but on my new 04 bambi, there is a shroud for the stove/oven exhaust on the street-side. In this shroud has a horizontal flap that covers the exhaust opening when not in use, to keep bugs, drafts, and crap out. I also noticed two little lever things that will lock that flap in it's close position, most likely for storage.
So if your hood vent is not venting well, it just might be the flap is in store mode.
Sometimes I wish I were living in the stone age. Then I would know I'm the smartest person in the world.