There are actually a few different reasons this could be happening. First, check your filter. Make sure the filter is in good condition and is able to still absorb and hold water. If the filter is extremely hard or getting dried out all the time, this would indicate it is time to replace the filter. They generally last about 1-3 months, depending on usage and the quality of your water. Second, check the humidity level in your home. This type of humidifier is a self regulating evaporative system meaning it naturally adjusts output to the amount of moisture the air can hold at a particular temperature. If the humidity level is already at 60% or above you humidifier will emit very little moisture into the room air.
As long as the fan in this unit is running and water is flowing through the water feature on the top grille of your humidifier, the machine itself is working properly and the issue lies with the filter or environmental conditions. A hygrometer can be picked up at a local hardware store so you can monitor the humidity level in the room and better understand why this happens.