How can I figure out why half my house is cold?
If it is a manual damper, there is usually a wing nut that attaches the handle to the shaft of the damper. If this wing nut is loose, it might allow the handle to turn without actually turning the internal damper. I would verify that the nut us tight and when you turn the handle, you can actually see the shaft rotating at the same time. If the tube of the duct has somehow been compressed, then the damper would bind up and not work properly.