About 1/2 dozen of my itti bitti d'lish AIOs have stains on them (of the number 2 variety

).
I have tried washing them numerous times, they have sat outside in the sun for days (in the rain too!) and I have even done a one off overnight soak in napisan, but the stains are still there.
Does anyone have any ideas about how I can get rid of them and avoid stains in the future?
Currently with number 2s I rinse them and give them a scrub with a brush, then I dry bucket them and wash them every other day (daily in summer) with ecostore powder or persil then they are line dried in the sun (weather permitting).
Tips anyone? I have tried liners, but DDs poos are too liquid...