The answer to 'air duct sealing not working' is that there are several common causes, including improper installation, leaks in the ductwork, and issues with the sealant itself. Symptoms may include poor airflow, increased energy bills, and uneven heating/cooling in the home.
To troubleshoot, first inspect the ductwork for any visible gaps or cracks. If you find any, you can try sealing them with a high-quality duct sealant or mastic. However, if the issues are more widespread or the ductwork is in poor condition, it's best to call in a professional HVAC technician to properly assess and reseal the system.