I make my own fragrances for my Lampes Berger using 16 oz. of 91% isopropyl alcohol, 8-10 drops of a pure essential oil, and NO water. I have had no problems with the stones going out.
I don't understand why some recipes recommend adding distilled water, but my Lampes work fine without it. Personally I think it's the water that may be fouling the stone and causing it to go out.