 Because it sounds to me like your system is unbalanced. You don't have supplies and returns that are working together to even out that airflow so that you have the proper temperatures and the proper spaces. And that is the first place I would look. Homes heat and cool not just by blowing air out of the registers, but by pulling it back and reconditioning it and reconditioning it and reconditioning it, either reheating or recooling.