ok, that sounds like you may be headed in the right direction. It the case of my friend, the gate did not full close due to the armature. In your case, it may simply be that it does not fully open. The armature pivot points are usually greased with white lithium type lubricant.
Hate to say it, but you may simply need some lube in order for it to fully open. If there is too much resistance on the armature, the vacuum assisted gate may simply not be able to overcome it.
Another thought is that debris in the duct work at the point of the gate may be preventing the gate from completely opening.
I realize my thoughts on this are not very technical, but judging from what you describe, it would seem to me that something within the air gate has degraded since new. I can only think of an elongated hole in the plastic, debris in the duct.
I have never had mine apart, but can only assume it is at least similar to the F-series units.
Let me know what you find, oddly, this is somewhat interesting to me. (I know, I'm weird)