You did not by chance use a different brand of servo arm than the servo is did you. You can not mix servo arms on some servos. On standard servos, Hitec, Appolo and Hobbico are interchangeable and Futaba, DAD, GWS, FMA and Tower Hobbies are interchangeable but you can not mix any of two groups without slippage or fit problems. Alsom ake sure you did not use a metric threaded push rod on a 2-56 coupler or visa versa.