Check if the cover is on properly:

The first  thing you want to check is that the inner lid is not loose and it's been attached securely. (See illustration). This plastic cap is what keeps the extra condensation inside the unit. When it's not attached properly, this is usually the cause of the leaking. If the inner lid is not attached or not aligned properly.

Once the inner lid is attached securely, make sure the cover is on properly. To know  has been correctly put on, the dot/arrow on the lid should align with the dot/arrow at the base of the diffuser. (See Illustration below).

Again, if these instructions or tips don't resolve your issue, please let us know so we can send you a replacement.