1. Lack of maintenance: Regular maintenance is necessary to keep an air conditioner running efficiently.

2. Dirty air filter: A dirty air filter can restrict airflow and cause the air conditioner to work harder than necessary.

3. Low refrigerant levels: Low refrigerant levels can cause the air conditioner to freeze up and stop working properly.

4. Leaking ducts: Leaks in the ductwork can cause the air conditioner to lose efficiency and increase energy costs.

5. Faulty thermostat: A faulty thermostat can cause the air conditioner to not work properly and not cool the room as needed.

6. Clogged drain line: A clogged drain line can cause water to back up and damage the unit.

7. Compressor issues: A failing compressor can cause the air conditioner to stop working altogether.

8. Electrical problems: Electrical issues such as blown fuses or tripped circuit breakers can cause the air conditioner to stop working.

9. Leaking refrigerant: A refrigerant leak can cause the air conditioner to lose efficiency and increase energy costs.

10. Noise: Unusual noise from the unit can indicate a problem with the air conditioner.

Regular maintenance can help prevent many of these problems, but when they occur it\'s important to get it check by a professional.