Umm... are you using standard carbon steel balls in your STNG?
IE you haven't "upgraded" to chromium balls?
If you have upgraded to chromium; they are inclined to create and retain a magnet field and may be attracting to themselves in the trough. I know STNG doesn't have magnets; but its something to consider.
I haven't had any issues with carbon steel balls in my stng. It seems odd that you'd have to shim anything... unless your trough has damage or your balls are magnetized.
My 0.02usd