We live in a house that is 5 yrs old. With the A/C on, the main level of the house is very comfortable yet the upper level where the bedrooms are doesn't seem to cool down at all.
I'm thinking that it has something to do with the duct work which won't be an easy fix. Could there be any other cause?

Is there a basement if so have you checked to see if the vent deflector deals are open or closed. As you walk the line of the vent you will see little handles and you can open and close off certain rooms in the house checck that but it would be good to have a second person upstairs while you are downstairs or there's a lot of stair climbing in the near future for you

You just might need another system for the second floor. The duct work is not the problem - having the big house is the problem and your a/c apparently is not large enough to properly cool the whole place. If the house is only 5 years old, how many sq. ft. is the main floor and the second floor?

Has this been a problem for the 5 years you have lived there or is it something that just happened or changed lately?

The problem has been the same since we moved in 2 years ago. We have a central air conditioning unit that will cool up to 2400 sq feet which is a little more than what we need. The deflectors have been closed so that the basement has no cold air going to it and the vents on the main floor and upper floor are all open.