My master bedroom runs about 3-4 degrees warmer than other rooms upstairs. I'm only about 1-2 degrees difference in upstairs/downstairs register temps (which is pretty good for a two story in texas with 1 a/c unit). I noticed that I have very low airflow from one of the two registers in the master bedroom. Air temp is also 6 degrees higher than other register at other end of the room (68 deg vs. 62 deg). Went up into the attic and noticed that the duct run for that particular vent is probably 25-30 ft long! I have no idea why it is that long, but could that be the culprit? Can a duct run that long pick up that much heat and lose that much velocity (CFM)? It is regular 8" flex duct, r-30 isulated attic. I haven't checked for kinks or collapsed areas in duct yet. The run from the main trunk to the register in question is probably 15 ft max. If it is just too long is that something that I could easily fix myself?

Thanks for any suggestions.