I have a room that I use a a guest bedroom in our home. As we don't have guests all the time, we keep the door closed. This room has three exterior walls, and one wall is probably about 70% windows.
This room is always much hotter or colder than the rest of the house, depending on the season. Currently, it's about 35F outside, and measuring the temperature of the walls and objects in the room shows the room is about 48F. The rest of the house is about 67F, and a bedroom just on the other side of the wall measures at 65F.
So this room is currently almost 20 degrees colder than the next room over.
I've had a blower test done, and the insulation and sealing came out OK. Our furnace works fine, but this room is all the way at the end of the ductwork. There's actually two ducts in this room (every other bedroom only has one), with one being just a branch of the ductwork going to the "primary" duct (the one near the window).
I have a ~17 year old single stage forced air gas furnace and air handler, it's oversized for the home, and has no trouble blowing heat around elsewhere.
I'm hesitant to throw more money at unnecessary HVAC inspections, but am willing to try to get someone to come look at this and maybe help solve the problem. I'm looking for recommendations of what to ask someone to look for, or anything I can look for myself - I've tried looking for "leaks" in the duct in the attic, but I couldn't find anything obvious.