I have seen posts for similar issues, but they don't seem to hit my issue. We have a single vent in our 3rd floor bathroom with no air coming out. All the vents on this floor are either on the floor or on the lower half of the wall. However, the vent in this bathroom is in the ceiling. I went in the attic above the bathroom and determined that there is zero air coming up through the duct work that leads to this vent. I could see down about 10 ft and saw no obstruction. I even got some fishing tape and was able to fish it down 15-20 feet before it got stuck.....not sure if it is kinked or could not make it around a turn through the duct work. Either way.....just looking for your thughts. Also note, we have one furnace and the dampers are open on the two main square pipes coming off the unit. Thanks.