Make sure the servo arm is 90 degrees (square) to the direction of the pushrod when the surface is neutral.

Does the servo move the same degrees in both directions?

I'm not familiar with that transmitter. But if it has adjustable travel, make sure it is set to the same number in both directions (e.g. 120). With JR transmitters, you must move the stick while in the programming mode with the "travel" screen displayed.