[huesmann]

1) Are you venting to outside, or venting to room?
2) How high above the range top are the vent intakes?
3) How far forward of the intakes are the burners in question?

 

[huesmann]

you sure its not in recirculation mode ?

This is why I asked the question. Microwaves have 3 settings for the squirrel fan: vent up, vent back, and vent to room (recirculate). They usually come set to recirc and have to be removed and reset to vent outside. One would assume the GC knew that, but you never know.

 

[huesmann]

The damper should open close to 90º. It won't open all the way to 90º because if it goes past a certain angle it may get stuck there—need gravity to be able to bring it back flat. Do you have access to the duct? You could drill a hole and stick an endo camera in to see what the damper is doing. Or core a bigger hole and lay eyeballs on it. Then just patch the hole with the proper duct seal.

 

[huesmann]

If it really blows 400 cfm, it should. But I bet it doesn't. Also, OP, does your duct have a lot of transitions or bends?