Hello, and welcome versa01 !! I am not sure if we have enough info...but I will mention a couple things that come to mind.
Of course the first thing would be a binding of the linkage somewhere. Remove the control rod from the servo arm and MANUALY move the rod to see if you feel a resistance. Locate the source and repair. If none, this thought comes to mind. Your servo arm MAY not have been centered when the control rod was attached. Remove the screw and the servo arm by pulling it straight up. (you can leave the control rod attached but I would remove it from the arm). Set the trim adjustment in center for your rudder, on the transmitter, and turn it on. Turn on your receiver. Your rudder servo will now be centered. Replace the servo arm by pushing it straight down on the servo shaft. ( NOTE: WHEN TRYING TO FIND THE RIGHT SPLINE....BE CARFUL NOT TO MOVE THE SERVO SHAFT. The best insurance here is to leave the radio and receiver on when replacing servo arm). Install the arm as close to 90 degrees to the control rod as you can. Now..... since the servo is now centered you MAY have to work on mechanically centering the rudder. Re-attach your rudder control rod and center the rudder by moving the clevis in or out or whatever it needs.
I apoligize in advance if you know these basics already..... I was not sure from info supplied. Hope this helps.....lownslo