how to bloomery

im playing the standard terrafirmacraft mod (newest version, 1.20 i believe, no modpack).

i constructed it correctly. the handbook now says 2 charcoal per 100mb cast iron. when i throw in a correct combination of those and light the resulting bloomery block (with fire starter), it has flames on top for 1 sec. or so and then just drops all the input items. what am i doing wrong?