You need to program both side of the mix. If it's a JR or spektrum radio, you must hold the rudder stick to the left when entering the mix percentage. then hold the rudder stick to the right and enter the same number (e.g. 10%) for the right. When you are done the elevator will go up a little for both left and right rudder. One side of the mix may have a negative number. But if the receiver is turned on, you can see visibly that the elevator is going up for both left and right rudder.