Yes it is rare that you just cant get one to seal, I have only seen two and only one had to change the valve, and like you I service these regularly.
What's bizarre is the o-ring works by deflecting under pressure and basically jamming itself into the gap causing a seal, so a softer one should work better as it's not a gasket, but sometimes the poly ones do work where rubber fails, I can only conclude it's a tolerance thing?