We have been in our home 3.5 years and have a gas pack to cool downstairs and a heat pump upstairs. Our living room is 22' high, serving as the ceiling for the upstairs landing as well. Both levels are normally set to 74. Recently, during the daytime when the outside temperature is in the 80s and 90s, the thermostat reads a current temperature of up to 79, and it sure feels it. I replaced all of the filters, flipped the circuit breaker back and forth, and checked the batteries in the thermostat. What else could I check?