If your home isn't selling, ask your listing agent to get feedback from brokers who have shown the house. Also, consider hosting a open house for broker's only. This gives you an opportunity to promote the property and get ideas on how to improve sale prospects.
Change the filters regularly. Dirty filters restrict air flow, reducing efficiency and worse case, can cause the evaporator to ice up. Disposable fiberglass filters should replaced. Electrostatic or electronic filters need to be washed regularly.