Do you have rooms in the summer that are warmer than others?
Are those same rooms always colder in the winter?
This is likely caused by an imbalance in air flow. There could be numerous issues or causes, so we will just focus on the most common ones.
You have two story, single system home and your thermostat is located downstairs.
Warm air naturally rises and cold air falls, causing your downstairs to cool off faster, shutting off your system before the upstairs reaches the desired set point.
Install a zone system with a dedicated thermostat for each space. This will operate the system for the required amount of time in both areas to achieve the desired set point.
(A less expensive solution would be to install a smart thermostat with a wireless Zone sensor upstairs this will average the temperature and run the system longer to achieve a higher level Of comfort.)
Your house is consistently humid and uncomfortable even when you crank your thermostat down.
Most people think that the solution for an uncomfortable house is more cooling. It’s likely that a previous contractor installed a system that is too large for your home. The AC system turns on and shuts off too quickly. This prevents the system from removing the proper amount of moisture from the air, even though the thermostat reads the correct set point. This produces high levels of humidity and could promote mold growth in your home.
Have a qualified contractor perform a heat load calculation on your home to verify the existing size matches what is needed and make the necessary adjustments.
Your energy bills are high while your home is consistently uncomfortable.
Your duct system may have a significant amount of leakage which causes you to get less of the air inside your home that your system produces.
Have your duct system evaluated by a qualified contractor to establish if your duct system is leak free or if the problem is equipment related only. A significant amount of duct leakage on the return side will suck in hot, humid attic air and greatly reduce your systems ability to cool your home.
Most AC systems consume more energy than needed due to duct related problems rather than AC equipment failures. You can save more money by simply evaluating and repairing your ducting as opposed to repairing/replacing your AC system.