Larson has answered a few thing, so I am focusing here on your later question i.e. when can you say that your amendment is effective. In general , if your amedment is not working for all metals owing to several reasons as Larson explained you can calibrate your target soil or waste as no soil is supposed to have all the metals that you have selected for this study. Secondly, you can go with iterations in mixture used for amedment like in this case you are using only iron oxides you can add some more material there like biochar to find a perfect composite like in our to be published work we mixed alginate, MnO2 and activated carbon. Overall there is no single magic wand to decrease bioavailability of all metals in a single matrix.

I wonder what sort of iron source did you add into the soil. If it is an iron salts the iron undergoes oxidation reaction towards Fe oxide formation which give off hidrogen ions. Then the pH is to decrease. such decrease can largely affect ionic speciations and adsorption preferences of the adsorbants. If you could better describe your conditions it can be possible to understand possible reasons for such behaviour in your soil.