Some of your rooms are too hot or cold.
Inadequate air sealing or insufficient insulation could be the cause. No matter how efficient your heating and cooling system is, if your home is not properly sealed and insulated, you will not be as comfortable and your system will have to work harder.
Your home has humidity problems, excessive dust, or rooms that never seem to get comfortable.
Leaky or poorly insulated ductwork might be the cause.
Your equipment needs frequent repairs and your energy bills are going up.
In addition to the rise in energy costs, the age and condition of your heating and cooling equipment may have caused it to become less efficient.