You need to get a competent Hvac contractor to do a capacity check on the system, including measuring the airflow with a flow hood. From the capacity & airflow checks, perhaps they can find where hot air is infiltrating the duct system, or where it is blocked. A zoned system cannot overcome an inadequate duct system, if that be the case. Capacity check involves measuring wet bulb temperatures & using enthalpy figures; unfortunately, many contractors do not know how. Many contractors just like to say it is doing fine , or it feels fine. The only true way to address the problem is to take the measurements & go from there.