Inconsistant boiling mechanics

I have been boiling hooves for glue, it used to be that if the fire went out and the hooves were only half done then I could relight the fire and they would complete, then one day I noticed that they had reset, I assumed it was considered a bug that they were half boiled and I thought "shame they 'fixed' that". But then it reverted to the old way of not resetting, so I assumed I had made a mistake and it was actually a fresh hoof or something, anyway, today I logged in to find that the two hooves (yay a new shed), one of which should have completed as it was half done when I logged off and the fire had 10 branches, had BOTH reset. How much water is needed? There was 30% in the pot, 1.5l, I thought that was enough for at least one, perhaps that is the cause of my stubborn hooves. It would be nice if we could add more water without having to take things out of the pot.