Lots of variables for this. Age of the house? How much insulation is in the ceiling and walls. Capes usually have knee walls, are these insulated? How many vents for HVAC? HVAC capacity? How many windows and what type? All of these factors will cause the symptoms you describe.

Age of house 45. 5 new double pain windows installed 2008. 3 hvac vents (hot) 2 return air. Blown insulation in walls and attic in 2008.the knee walls I don't know. We can feel a strong draft coming down the step, enough to blow out a match.

Have a test and balance run. You have 3 supplies and only 2 returns. Make sure all supply and return registers are open and unblocked. Make sure any access panels through the knee walls into the attic space are closed, sealed and insulated.