trex 600 or 450?

in either case you need to examine the parts of the system. take the control horn off the servo and see if the rudder movement is free and easy to move to both limits. If it is sticky at the servo end of the control rod then you need to work towards the tail blades. check the control horn at the tail housing to see if it is sticking. if it is free then check the pitch slider, if it is good then check the ball links to the tail grips. If they are reasonable then check the tail grip bearings. If you found the control rod at the servo end easy to move to both limits then maybe somethings wrong with your servo? The Trex 600 has a known problem with the tail grip bearings. Most of them are ok but some have failed and caused the symptoms that you mention.