Smelter empty

[clouddog]

Not sure whats happening because its just the last 2 weeks or so it has happened but Happened 3 times lately, i have not figured out why.
first time i thought maybe i forgot to just add anything in it as i run multiple smelters.
second time i got suspicious and started checking every smelter 2 times before starting them,
worked out like normal for a while then today again it was empty after finishing. and this time i did doublecheck before starting it so i had like 13 ores in it and 12 lime.
noone else near, i was standing next to my smelters just as they finished and one smelter finished without any dross or anything in it,totally empty.
abit unsure what effects it or if its a glitch/intended.