Why is there no mist coming out even though the water tank is full and lights are on?

Several issues could cause this: the water tank may not be properly seated on the base, mineral buildup from lack of cleaning may block the ultrasonic transducer, a demineralization cartridge may need replacement, the mist level setting may be too low, or the mist outlet could be blocked. Clean the unit according to the manual and ensure the tank is properly positioned. If none of these solutions work, the unit may need professional servicing.