Is the defrost output tube plugged with anything? Try pulling hoses off to see if you get air to the place you hook up vent tubing.
Chris
I was going to pull it off and have a look, if only to try and push or pull that flapper valve; it's all brand new so "shouldn't be" - but never say never.
I really think it's the flap as it is sticky; believe it isn't travelling far enough (and that's why I want to know what direction is which)