Funny thing...is seemed fine all summer, but it doesn't get sun. It's a guest br so the door is kept open. I'll do a test with the open/closed door. Guess I should call a heating/air technician to investigate.

If it is not to temperature when the door is closed, but is OK open, more return air is needed. You can also tell if there is significant wind as the door gets close to closed. (use a facial tissue to see it.) That does not seem to be your problem.

I closed the door for an hour. When I went back to check, it was still very cold. Whether open or closed it seemed to make no difference.

Have you looked inside vent with flashlight to make sure it's fully open? Have you felt to see if air flow is same as other rooms? Have you checked at unit to see if dampers are installed on ducts? Have you checked attic insulation?

I really appreciate your time and suggestions. The vent is fully open and airflow feels strong. I have not checked at the unit at all. Would need someone to show me what is what. Can't access the attic. There is only a small hole/panel that's painted shut (no stairs) in the hallway ceiling between the two bedrooms. If insulation was the problem wouldn't the other br be having the same problem?

Sometimes the insulation contractors do a great job and sometimes they don't. You can have gaps in the insulation. If the roof line is low over that room, maybe someone got lazy. It could be that the AC contractor did not properly do the calculation for the air flow needed for that room.

Sometimes dampers are installed on supply ducts to balance air flow. It costs, so it is not always done. If you can access the furnace, you can look for levers on the duct system. Do an image search and find a few views.

Here is something that you might do. If all other things are equal, the area of the supplies (Square inches generally used) should be proportional to the area (square footage generally used) of the rooms they are in. Since this room is over unheated area and you mentioned two large windows and two outside walls. I am guessing this is different from the other two bedrooms that have maybe one outside wall each and may be over finished area.

The other side of the coin is that if this bedroom is similarly sized to the others that have one outside wall and are over finished areas the supply for this room should be bigger in the cool room than for those.

Thinking of solving the problem, the easiest and least expensive thing to do might be to add insulation under the floor since it is accessible. If the attic is open and easy over that area, that might work too. If there is already a lot there, however, it won't do much good and you will have to look elsewhere for significant improvement.

You are correct that the other br (on the same side of the house) only has one outside wall, but both of the rooms are over an unfinished part of the basement. RE: insulation under the floor- there is insulation on all unfinished ceilings in the basement. Also, I looked for levers, but didn't see any.

I think I'll contact the previous homeowner and ask how they handled this. Luckily, it's only a guest room and we don't usually have this much cold weather in Georgia.

Just look at the rooms and evaluate the heat loss seat of the pants method, windows, outside walls and floors, square feet,..... If the cold room has what looks like higher demands, but has the same or less air supply, judged by the size of the supply register, that is a good indication that you need more air there.

If the register size is proportional, it does not mean that something else cuts air supply to that room, but if the register appears smaller than it should be, you have a good indication of trouble there.

Both rooms are the same sq footage and have the same size register. The difference is the windows are 10" longer and there are two outside walls instead of one.

I'm going to buy a thermometer :)