I took off the top centre vent cover on the top of the dash(retained by only two screws)and you can see a flap.
Moved the vent dial with just the ignition on, could see it was trying to move but was sticking.
Sprayed some silicon grease on my finger tips and greased the edges of the flap both sides, tried it again and it now moves freely.
Hope this helps, seemed to have worked for me.
Good news! I took it apart and it is that!
Bad news! No amount of silicone spray in the world is getting that thing budged once it's in the 'UP' position!
I have to physically move it down by hand for it to move, otherwise it will stay up. Like you said, I can see it trying to move down but it won't, and once I've moved it down it will move back up on its own just fine, it just won't move down on it's own,
When I move it down there's a touch of resistance, then a click, then I still have to move it all the way down by hand.
EDIT: I've now realized what's causing the fault. It's as though the radio or something is set too far back, so the fan blade is snagging on it and stopping it from dropping down. How very odd, I've tried pulling the dash front out a bit to no avail, I think I'm gonna have to get the radio removal prongs and pull the radio out a bit.
Has anyone had this problem before???