That's interesting perhaps it's not in the right form for it to be absorbed too. Apparently ph and eh can affect it's availability and certain rhizobacteria. Potassium makes it more available too. Perhaps some oxidised hot compost which you added some wood ash too might be worth trying. Certainly would raise your ph.

Going try and foliar stuff next year, most things were not that bad here despite it being very wet....perhaps the em rhizobacteria helped but the hostas got hit badly by them so that would be a good test for the iron feed.