We just bought our first house, a ~1925 bungalow in the Chicago suburbs with a finished attic. The main level and the upstairs (formerly an attic that was finished into two bedrooms at some point) operate on their own HVAC systems and thermostats. Though we've only been in the house for a month, we've noticed that our main level never really cools down in the evening while our upstairs often cools down appropriately. Granted, the main level is larger and has far more windows and furniture while our upstairs is pretty empty at the moment, but it seems odd for our main level AC to constantly be coming on and off throughout the night to hit 74 while it's in the 60s outside in the evening, and our upstairs is a cool 68-70 in the evening.
Any ideas? Bad thermostat? Maybe with all of the windows, even with the blinds closed, the house is just retaining lots of heat throughout the day on the main level?