Hi Dear, The outdoor unit compresses the gas at high pressure and will pump it to the indoor unit and the gas will get decompressed in the indoor unit creating a drop in temperature. Due to regular wear and tear there maybe some leak which cause the refrigerant gas to leak resulting in inefficient cooling. Sometimes the issue may be with the compressor in the outdoor unit. Sometimes thermostat malfunction also cause this issue. You have to contact someone who really know about A/C to get it fixed.
