Cary, from everything that you describe you are doing it perfectly.
You obviously have experience with this tool. I use my stretcher for
so many things...
I hesitate to blame a tool for a job gone awry, but perhaps the
milling is off on the reduction dies. Do you still have the old
stretcher or a friend who has one? If so, compress a ring on another
machine to test your technique. Also test another ring in your
machine. Once you -- or the ring-- have been eliminated as variables,
the culprit must be the machine itself.
The only time I have had problems is some uneveness in the band,
such as a solder lump or uneven annealing was the problem. I am often
suspicious when a new tool yields problems that an old tool never