The most likely reason is it is old. Sometimes there are issues with installation, or the quality of the equipment, but usually when HVAC systems start to break down it is a sign you should begin looking into a new system, that will save you more in the long run. Other issues may be caused because of dirty coils, or improper refrigerant levels with will cause the system to work less efficiently. Having a properly sized unit is also important. A common problem for our technicians (with 40+ years in the business!) is many people think a bigger unit will cool their home faster. This couldn’t be further from the truth. A large unit can’t dehumidify your house like a smaller unit, and it will turn on and off more frequently costing you extra money.