Have you tried just bare servos? Measure the movement, see if it does equal movement with stuff out the plane.
If it’s fine out the plane then you know the linkage on the elevator is the problem. Start by checking that the linkage stopper, if used, is moving freely. Then check the hinge is free to move, if you put too much hot glue in the himge bevel it can stop the smooth movement of the servo. If the hinge moves freely and linkage is free moving then check the position of the linkage in the servo horn. Nearest the plane = most movement. Also the position of the linkage in the servo horn, further out on the arm = more movement.