Aircon that was installed is not working

We bought an aircon and had it installed by Mikah Air. They installed the unit and then that evening when we tried it out it was not working. They came out the next day and claimed that nothing was wrong and it was suddenly working. A couple of weeks later still on the same setting the aircon was not effective. We gave them a call to come out and check the unit. They claim that there is nothing wrong with the installation and that we do not know how to use the aircon. No settings were changed.