we ran into the same thing
we are using single wheel shooter with a stationary pinch bar to compress the ball into the spinning wheel makes for lots of backspin. The video link is what we found with the worn balls vs. new balls. the issue seams to be the compression reaction more than the surface wear. the new balls will take effort to move the air out of while used balls have lots of little nicks and cracks for the air to escape out of thus the force to compress is much less.
Our solution was to reduce the amount of pinch on the ball this limited our distance but greatly increased our consistency from one ball to the next.
http://www.youtube.com/watch?v=4EueP...eature=channel