Do you ever do washes with hot water and Chlorine bleach? Not the diaper washes, just other washes.
I am surprised that the following does not work.
Diapers and baby clothes get a cleaning boost when soaked and washed with 20 Mule Team Borax. Helps get rid of odors, reduces staining and makes diapers more absorbent.
Sometimes after washing once some people wash again without any soap etc to give an extra rinse.
Google cleaning diaper basics
It is possible that something in your water is counteracting your cleaning agents.