Reduced mist output. Over time your ultrasonic disc produces less mist. The change is gradual, until one day it can no longer atomize water and oils effectively.

Quick flash, then nothing. Some units use the disc to detect when the reservoir is low on water. A faulty disc confuses the controller and shuts the unit off immediately.

Water just ripples. Ultrasonic discs have a lifespan of about 5000 hours. It is a physically moving part, so think of replacing it as routine maintenance.

Nothing happens. This could very likely be the disc or the power supply. Try a different power supply from an identical unit, if possible, before purchasing a disc.