My experience is with VariX IIIs and above.

Root causes in order:

Bad Mounts such as Redfield JRs.

Bad shooter such as a few here on the fire. Practice more.

Bad rifles

Bad scope. The MK6 was more prone to real optic failures than anything else I have seen from Leupold.

I had a VariX II long ago that worked great as set and forget.

I had an early VX-6 with measurable track out during zoom but it held zero at max zoom.

I have tested and shot a few hundered VX-6 and VX-6 HDs.

John Burns

I have all the sources.
They can't stop the signal.