The level is going down, but you can’t see it !
The diffusion of the liquid from the Diffuser is a very slow process. When used normally it is difficult to see any change in the vial during the first week of use. Slight variations will occur depending on air turnover.
To check that your diffuser is working properly, make sure that:
the wick is not broken
the device is left turned on (sometimes an electrical outlet is controlled by a wall switch)
finally, try the device in a different electrical socket.