just learning out loud here......it sounds like there is a threshold for which conventional softeners dont work on iron, simply put, in my case, i have ~2 ppm Fe with small visible oily blebs on the hardwater (insoluble Fe) that our conventional softener DOES a good job on......so we must be right at (below) the threshold....so probably > 2ppm with insoluble Fe, and you gotta do something different, like others have suggested above.

GSF are people too!