There is some kind of screen/view protection on your tank wall. Obviously this blocks the sensor. Any kind of metallic or metal based coating will disturb the measurement (also metalized glass)

how can I fix it?
If it´s working on bare glass, you can try to remove the screen. But for sure, this may not be an option due warranty or the hardness of the coating - in this case you can´t use a contact-less sensor.