Hey Magicman!
Welcome to the forums! (and yes this is the right place)
Couple of things can "gotcha" here, so we'll start with the most typical and work our way down.
With your surface at neutral (flat with the rest of the wing) is your control horn hole lined up with the hinge? If you made a line from it to the hinge it should be at 90 degrees to the paper when neutral. A little forward/back and you loose throw to one side.
If that's not it, the next thing to check for: Is your horn or control rod bending more in one direction? The shouldn't bend much either way, but the hinge may be stiffer when you bend it inward if the bevel isn't tight (you're crushing the foam in the hinge), or tape/glue is getting in the way.
Finally, check your servo horn. When it's centered, is it pointing 90 degrees from the fuselage line? The servo will normally throw 90 degrees (45 CW, 45 CCW). If you have the horn pointing too much off 90, the forward back motion you want is cut off as it starts to swing around the circle. You'll get the most back-and-forth throw if it's centered at the middle of that circle front-to-back, or 90 degrees off the fuselage.