This is normal and means your water management system is working correctly. Evaporative cooler systems are designed to monitor and control the quality of the water in the tank. So, it only drains water when the system really needs to. The evaporative cooler does this by bleeding off water down through the drain, which is replaced with fresh water via water inlet float valve as the water level in the unit drops. The frequency at which the water manager bleeds off is determined by the quality of the water in the tank. A probe in the tank measures the water salinity (impurities) and, when this rises above a set level, the unit will drain some water.