What would cause a Delta to COMPLETELY lose all of it’s calibrations from Day to Day?
Three Days ago, my M666 line to get it happy was:
M666 X-2.1 Y-3.2 Z-8.
And it really wasn’t consistent between homes
Yesterday, I Got serious and got everything down to:
M666 Y-2.2
Now, with those same settings, Everything is WAY off again, and it looks like another 15 minute session to get it all dialed in again.
Oh no, wish I knew more to help 
Did you M500 and save over them?
No, I was adding it to the Print Specific g-code in the slicer, after the G28 command. (I suspect I’m doing it wrong? 
)
You have belts? How old is the printer and which design.
Filament, and it’s been pretty stable, as far as I could tell, with no big crash events that would cause it to stretch the filament.
Fwiw, it just makes a big nasty first layer, then the second perpendicular layer makes up for most of it, but still, it’s kinda frustrating when something I thought I’d licked comes back to haunt me.
Spectra? it stretches over time. Best to pre stretch it before use. Once stretched it should last a while. Initially I have to tighten mine every day and re calibrate, now it rarely needs recalibrating.