More than likely the air conditioner is undersized and/or working harder to keep to the desired indoor design temperature. An air conditioner works properly and efficiently at the temperature it was designed to achieve during installation. If there are hotter than normal days, the A/C will try to maintain the indoor design temperature. This does not necessarily mean there is an undersized unit. It means that the particular hot day is outside the normal range of the calculated design for the A/C unit. A proper load calculation from an air conditioning contractor can determine if the air conditioner is properly sized for the geographic location.