I kept glaring at my smith wondering why he wouldn't smelt iron ingots even though the menu says he can. So I figured out why. Essentially, I thought I had enough ingredients because iron ingots production s not greyed out, but in truth I didn't have enough ingredients. I realise that it's consistently doing this for specific items, iron ingots is one, sausages is the other. Basically, the formula for greying out the production of iron ingots should require at least 3 hematite ore and 1 coal, but it only greys out when you don't have ANY hematite instead. I'm guessing the devs changed the formula how many units of hematite ore is needed to produce but didn't change how many it needs for greying out.