Why is the upstairs of my house always hotter in the summer & cooler in the winter?

Why is the upstairs of my house always hotter in the summer & cooler in the winter?

Heat rises, thus the reason it’s hotter in the summer. Assuming the system was sized and installed correctly, you should consider installing a zone system if possible. Sometimes the addition of return air ductwork will help improve air movement and help make upper levels more comfortable.