The answer to 'preventive maintenance not working' is that there can be several common causes, including neglected maintenance, improper maintenance procedures, or issues with the HVAC system itself. Symptoms may include reduced efficiency, poor airflow, unusual noises, or even a complete system failure.
Some potential solutions include ensuring regular maintenance is performed by a professional HVAC technician, checking and replacing air filters as needed, and inspecting the system for any mechanical issues or wear and tear. However, if the preventive maintenance is still not resolving the problems, it's best to call in a professional HVAC technician to diagnose and address the underlying issue.