Inspected a Home yesterday where I got a temperature of all bedroom vents (farther-est away from furnace) 81 degrees when all other vents where getting at least 120 degrees.
Just wondering for my own knowledge what items could cause this: closed / partially closed damper in the duct, broken, disconnected, big hole in duct, etc… Anything else?
Charley or anyone else thoughts?
Thanks
Airflow balancing.
Is it an old house? I some times find duct work up the wall and ends at the 2nd floor to just run in the joist bay (no duct work). When I see this I start scanning floors to find the joist bay being used, 2 homes this week alone were that way. I always confirm by popping the diffuser
Is the duct work running though a crawl space or a cold area? Sounds like a balancing issue as Michael said. Do you also measure CFM?
Where was the furnace? Home owner tells furnace installer to put it at the farthest end of the basement so they will have lots of room to develop the basement without that annoying furnace room getting in the way.
Duct designer wants to place furnace in the middle of the house so all ducts are more or less equal length.
So who won this battle? 
As noted above it may be possible to correct with air balancing. It may be possible to get more heat to the cold rooms by partially closing dampers in the hotter rooms, but there is no substitute for properly designed and sized duct work. The bedrooms should also have a return air grille, if there isn’t a return air in the bedroom, a grille can be installed in the door, or they cut an inch or so off the bottom of the door.
Answers:
Furnace was located in the Garage.
Bedrooms on other end of Home from Garage.
Home is single story home.
Registers in floor (ducts in slab)
Up flow or down flow furnace, slab foundation or raised??? I answer questions with questions;-)
Whoops I did not read enough of the post.
The ducts are not in the slab they are beneath the slab and this time of the year it takes a good bit of time to heat the ducts the first time you fire the furnace if the home was vacant. Your gona have temp duct loss depending on the distance. If you don’t measure CFM you could at least feel the air flow and compare it to like registers to use the SWAG system. I have only discovered one separated duct beneath a slab in all my years but not to say its not impossible to find
I have seen underslab duct get water in it. This would extract a lot of heat by the time it gets to the register.
A leaking duct can not be balanced…