For the first attempt I dropped Rich magnetite ( A full stack, the rest dropped out which was around 28 pieces) and a full stack of charcoal (same here, used a full stack and collected the dropped rest) - The next try I did with charcoal first and rich magnetite, then I used normal magnetite and charcoal and then charcoal and normal magnetite again
(again, the last two attempts with full stacks, and then I collected the rest that dropped out)
There was also a problem right after building- and rebuilding the bloomery, and that is, that the first times I dropped magnetite and coal in there, I saw the unburnt slag appear, only for it then to dissapear a second later and dropping everything again. After that, it took the items normally (although, without producing an output)