Understanding the mechanics of the cooler will help you improve its performance. Evaporative air cooling works by drawing in hot dry air, and fanning out cooler, more moist air. After a while, though, your room will need ventilation or a cross-breeze to circulate the more humid air out the room and restart the process with fresh dry air. In a very large room, it doesn’t necessarily need air from outdoors, but it does need some type of airflow to keep the cooling action moving. Therefore, confirm there is adequate cross-ventilation.
Another performance booster is running cooler water through the air cooler. You can boost the coolness by adding ice into a dedicated ice compartment (if the model is equipped with this) or directly into the water tank – but only if the ice does not hinder the pump or its circulation.