I had the same problem on my 2017 Citation24SR. I did a lot of work and got the ducts to blow air.
First I took the ceiling sky light out and could access the duct work which goes to the front vents. The forward vents use 3" round tubing that connects to a manifold right where the ceiling turns upward. The ducts where leaking as was a few places in the 3" tubing due to holes. I patched the holes and sprayed foam into the ceiling around the manifolds. The holes or gaps where so big the foam went into the normal ceiling ductwork prior the manifold. Next, I added two ducts in the ceiling just forward of the AC unit. Once I figured out where the air ducts where which went forward it was easy to just cut the holes and add the ducts. While I had the holes open I was able to cut and clean out the foam which sealed around the manifolds. I also noticed many places where wires went through the ducts which left holes in the duct work. I used aluminum tape to seal all the holes. I next took out all my ceiling lights and to my surprise a lot were within the ceiling AC ductwork. Again taped all the gaps I could find. I also plugged the ductwork behind the vents over the bed since the air shouldn't have anywhere else to go except out the duct by that point. I also looked at the over bunk vents. The manifold there is large and what air pressure you have gets diluted by the large manifold so I reduced the internal size of the manifold with some flashing cut to size, taped it in place and painted it black. Lastly, I taped everything within the AC blower area in the ceiling to make the system as tight as possible. In the end, I get excellent airflow out of the new vents in the living area and twice as much air now out of the forward bunk vents. Its not as much as the living area but its at least 2-3 times stronger airflow now. Don't understand why Thor doesn't add two more vents in the first place in the living area since its so easy to do. Don't have pics of the work I did but do have one of the new vents.
Jon