This is a long shot answer, but something to check. How far are you moving your rudder?

On one of my planes several years back, I was trying to get max rudder movement, but the hinge line wasn't beveled that much, so in trying for max movement, I was actually pulling the hinge loose with the servo.

Not wanting to change the bevel and re-paint, I had to re-hinge with a looser hinge gap to get the movement.