Looking around on the web, I am finding that soluable iron will get trapped in the filter resin and the normal filter backwash doesn't wash it out. There are several products such as Iron Out and Rust Out that are supposed to remove the iron and other debris from the filter resin.
I have noticed lately that my water softener (10 years old) has been using a lot less salt lately and water useage has not decreased (it has probably increased). I do not have a prefilter on the unit, water comes directly from the well tank.
I just picked up some Super Iron Out today so I will try it out.
When the PS234 kit ships out I should be able to analyze the water going in and coming out of the filter. That should tell the story.