It sounds to me like its getting too hot. Most commercial fog fluids are 30% to 70% distilled water and the rest PG / VG. As you add more water to PG the flash temperature goes up. If the unit is designed at too high a temp, you can get problems with things actually burning. With organics, this normally means you make tar which would build up and make a mess.
Try adding more water in some juice or possibly lowering the voltage slightly to see what happens at a lower temp.
Try adding more water in some juice or possibly lowering the voltage slightly to see what happens at a lower temp.


