yeah, like HB said, the forge is a bit different in that all of the ingredients must be smelted into the forge first. There should be a counter to the right with how much of each material is smelted. Typically, players will go on dedicated mining trips and immediately place large amounts of material into the forge to be used when needed. So, whatever excess iron, lead, brass, stone, clay, that you have, just pop it into the forge. (I rarely smelt sand as I don't craft glass and need it for concrete production).
Smelted material is not always 1-to-1 conversion, either, as I recall. Having an Advanced Bellows in the forge will makes things smelt faster. Having an Anvil will make things craft faster. Finally the Crucible (mid-late game item) will allow you to craft things made of steel.
Also, when it comes to smelting, it doesn't always have to be raw materials that you put into the forge. For example, you can place tools like hammers that you no longer need into the forge and that will smelt for a larger amount of iron. That is sometimes more valuable to you rather than selling low-quality items to the trader.