1) Both the humidifier and the drinking water bubbler.
The most likely reason is a bad seal between the tower and the AquaCure. If gas escapes from the tower, it won’t travel down the tube and into the humidifier. Retaping the metal threads, and ensuring the black o-ring in place between the tower connector and the tower (if you have the new model tower), will normally solve the issue.
2) Just the drinking water bubbler.
This occurs when there is a gas leak between the humidifier and drinking water bubbler. The lid on the humidifier is very easy to cross thread when screwing it on. Try removing the lid and screwing it back on, paying special attention that the lid screws on level.
If this doesn’t work, please check that your hoses are tight and that the humidifier lid is not cracked.