How can I get my upper level to be as cool as the lower level?
Here are my generic suggestions based on a few decades of home ownership in a climate that routinely ranges from -30F to 100F with dew points approaching 70:


Fix up your insulation. Cooling is usually more expensive than heating, and while hot air rises, heat does not. A hot attic with poor insulation means a hot ceiling, which means radiant and conductive heating of your living space. 
Be sure you have appropriate window treatments. Dark-colored blinds or curtains absorb radiant solar heat and conduct it to your living space's air, as well as re-radiating into the room. 
Balance your system seasonally. In the summer, your lower-level supply vents should be mostly closed. Your returns will draw air from throughout the home and keep things circulating, and cool air will naturally drop from the upper level. You don't need much (or any) conditioned air feeding to the lower level.
Use ceiling and portable fans to augment HVAC airflow and improve human comfort. Moving air evaporates sweat much more effectively.
Have reasonable expectations. You may not have been trying to achieve 72 degrees as an indoor temperature, but that's quite cool. 76-78 is much more attainable on very hot days, and generally feels comfortable with adequate airflow.