Two Ultrasonic Foggers 24VDC - Express Postage Included.

These foggers have a ceramic disc which vibrates at very high frequency.
The `sound waves’  break the liquid water into airborne fog particles.
Water, if not close to pH neutral, (and pH is a log scale – E.g. 8.0 is 100 times more alkali than 7.0) will throw a cast - calcium if the water is too alkali, or rust if too acid.
The ceramic disc rim becomes fouled and the disc starts to vibrate off-frequency then stops.
We say on the label of our machines and in the book to monitor our water and treat it if it’s excessively off-basic.
Tank rain-water contains whatever dust settles on our roofs. Town water is treated only for contaminants not for its pH.
And of course, yes, you may have perfect water quality, used distilled water even, but a puck is an inexpensive consumable item with an uncertain service life.  

Try first soaking it in a CLR solution or vinegar, and a soft toothbrush to clear the fouling.

We nomally have replacement pucks as a two-buy. But an alternative is to buy these pucks from eBay, where they are available with FREE POSTAGE, cheaper than what we can do it - see example below $14.44 each if buying four. Make sure they are 24VDC, 20mm Disc and that they are posted from within Australia.