No cold air (or warm )coming from center vent
 

All 3 vents seem to work fine except for the center one closest to the driver.
I tried to adjust it with the control unit but still no luck.
I can live with it but I wonder if anyone knows how to fix it.

Many many articles on this one. Very common. Check all the adjustments to the vent itself and if it is indeed open, you will want a VCDS scan to check for codes. Possible failure of the vent flap is my guess, but it's hard to be certain without further investigation.

And it's no fun to remove the mechanism to access to repair/replace. See the service manual. Or, search as Mishar and, I believe others have posted a drawing of the assembly. Most of the dash has to be removed IIRC.