Hi there,
I'm not a CFS'er, just a caregiver, but I did have quite a challenge with low ferritin many years ago. Intravenous iron made an amazing difference very quickly (though it turned out I had an allergic reaction to the carrier so we could only do it one per year for two years). So far as we can tell, my low ferritin was very likely caused by truly ghastly intestinal parasites - as in worms. No kidding.
I went through all sorts of diagnostics 'cause no one could figure out why, then a few years later, with chronically low ferritin, my naturopath discovered and got rid of the parasites. My ferritin recovered and has been fine since. If memory serves me, my naturopaths research was that particular parasite loved iron, and was gobbling it up before my body could absorb it. I can't prove anything either way, but no problems since, after about 5 years of low ferritin seems interesting.