If you have been using your air conditioning lately because of a late-winter heat wave, such as those you get in the southwestern regions of the country, and you've noticed that one room in your home seems to be a lot warmer than the others, you may have been checking the usual suspects. Weatherstripping, exposure to direct sunlight, and more can all affect one part of your home. But if those don't seem to be at work, you might be baffled as to why one room is not cooling like the others. There are additional reasons, and they are easy to deal with.
Check the Vent
It's a small thing, but many times the dampers on the vent in the room just aren't open all the way. You'll see a small lever on the side of the register; pulling that lever down usually closes the dampers, and pushing it up opens them. Make sure the dampers are fully open and see if that improves air flow.
Check the Duct
It's possible the duct leading to the room has become clogged with so much dust that not a lot of air is coming out. Grab a flashlight and peer inside the vent opening through the dampers. If the duct looks very dusty, you may want to have it cleaned out, even if you can't see an outright blockage. It's normal for ducts to have dust on each side, but if it looks positively snowy in there, have the duct cleaned out.
Check the Ceiling Height
Warm air rises and cold air falls -- and that could be causing the odd temperature difference in your home. Compare the ceiling heights in all of the rooms. If the room that's too hot has a regular flat ceiling, and the rooms that feel fine all have vaulted or cathedral ceilings, you have your culprit. What's happening is that in most of the rooms, the cold air from the air conditioner is falling down, to where you are, and the warm air is rising far above you, making it seem much cooler near the floor in those rooms than in the room with the regular ceiling. In that room, though, the warm air can rise up only so far, which isn't that far away from your head. You may want to leave the door open in that room when possible to let cooler air from the rest of the house flow in.
If these also don't seem to be at play, contact an air conditioner repair company. The contractors there can take a look at the entire system and see where the flow of cool air could be blocked from getting to that room. Visit a site like http://www.smedleyservice.com for more ideas and help.