I'm pretty sure bringing the pH down was to get and keep the iron in solution to start then use HEDP to try to keep it in solution.

I believe the mechanism for removing iron with cal-hypo is to let the cal-hypo raise the pH causing the iron to drop out on the filter with the calcium precipitate. I would backwash before dropping the pH. Did you do it that way already or did you backwash after dropping the pH?.