A visual reference would be helpful. Can you record yourself doing this in slow-mo?
That will require replacing the fly line in the reel with mono and rigging up a shooting head, and the recording...I'll see what I can do for you. No promises.
But not to leave you hanging - here's a vid of Mike Kinney showing how to reduce rebound by a big margin:
Lot of casts in the clip, including some single-spey casts. You can find and read his posts here about single speys done with shooting heads and weighted flies.