A swamp cooler works by evaporating water that passes through slats or filter-like media. If insufficient water is not passing through, or the air in the area is full of moisture it will do less work. Check to see if the media is clogged, preventing air or water from coming in contact with water. Slipping fan belts, slowed motors, or dirty fan blades will also slow the air. Cleanliness of the components is important.