A: Make sure there is enough water in the tank. It can be filled with up to 100ml of water. The water level in the tank will decrease as the end of the timed session nears. Alternatively, if there is too much water in the tank (more than 100ml), the mist will not operate normally.
Other possible factors
Is the ultrasonic diaphragm dirty? Turn the power off, then use a cotton swab, or similar, to gently clean any bubbles adhering to the ultrasonic diaphragm.
Are there any bubbles accumulated on, or around, the ultrasonic diaphragm? At low water temperatures, bubbles may accumulate on, or around, the ultrasonic diaphragm, preventing mist from emitting well.
Is the air intake at the bottom of the diffuser unit dirty? Dust accumulated around the air intake at the bottom of the diffuser unit will reduce air flow and prevent mist output.
Other causes? Mist may not be visible depending on the temperature, humidity, or other surrounding factors. Even if the mist is difficult to see, there is no malfunction as long as mist is being generated within the water tank and the fan is operating (you can hear the fan motor).