There are a few reasons why your air conditioner might be on but not cooling. One possibility is that the unit is frozen - if there's ice built up on the coils, then the air won't be able to flow through it properly. Another possibility is that there's something blocking the airflow, like a dirty filter. Finally, it's possible that your thermostat is set too high - if you're trying to use it in an area that's not hot enough, then the air conditioner won't work. You should call a cooling expert for help if you have any problems with your AC.