If your diffuser isn't misting, or has next to no mist coming out when it runs, this is usually because there is not enough water in the water tank. Make sure that the water tank is filled to the maximum level line.
Alternatively, the water level may actually be too high, or, higher than the max water level. To remedy this, just pour out the extra water and try your diffuser again.
It's also possible that the disc may need cleaning.