I agree with jb. Also consider fiberglass batt insulation under the floor. The cfm output of your furnace starts with the motor. A multispeed motor may be rewired to get more cfm but you need to have your hvac person look at that.
If you are leaving the doors to the rooms closed, then start keeping them blocked open a few inches or so for a few days to see if cutting returns in the doors will help much. I'm with jb on adjusting the dampers and insulation. If closing down dampers doesn't really move air into those rooms then you need to have an hvac person size your ducting and maybe you have room for another duct or larger ducts to the cold rooms. Just don't close everything down too much.