I had a Sig 25% CAP and it would KE loop just fine. I used a ~80oz @ 4.8V servo on the rudder and it too worked just fine. My rudder throw was about 45 degrees. I am now using 140+ Oz servos but it would KE loop on the 80 oz servos. I dislike and do not use the throw in inches measurement because it makes little sense. But I ran down to the garage quick to measure my rudder throw that way and it is over six inches measured at the bottom of the rudder... a lot more than your three.

So, there have been some good suggestions such as moving the CG aft and reproping. I'd try those too. But I think you just don't have enough throw yet.