The only thing I can think of that you haven't ruled out is some contaminant getting into the developer and poisoning it. I take it tanks etc. are thoroughly washed ... which still leaves the water. I don't know what your tap water is like but round here the quantities of chlorine (added as a bactericide) vary greatly. There may be other chemicals which get added which are (allegedly) harmless when drunk but might affect the developer - in particular alum, used to precipitate silt so the water looks clear. Try using demineralized water, or even cheap supermarket bottled water, instead of mains supply, to make up your developer.