Frawg - your results are the same as mine using the constrictor arrangement that ZA206 originated. I went from an 8" bury which slipped to a 24" bury which didn't slip. I noticed as you have, that with the 24" bury only a fraction of the bury near the end at which the force is applied was effective. The opposite end was usually loose and ineffective.
I then abandoned this method and adopted carboy's method which applies force to both ends of the constrictor. This makes the full length of the constrictor fully effective and a much shorter length can be used and no back-up half-hitches or any other knot is needed.
That is also why eye splices with a fully buried tail are so effective and are rated at between 97% and 100% of the rated rope strength. With the eye splice and buried tail, the force is applied to both ends of the bury.