Are you turning the "trim wheels" at all? This is a very common mistake people make. Those are NOT "trim" wheels at all, but rather "pot adjustment" wheels. You are far better off centering them carefully (into the detents) and then calibrating carefully in Game Controllers (Windows) making sure you have good, accurate deflection in both the X and Y directions. After that, calibrate again from inside AH. Another common mistake is to think that if you calibrate from Windows, that is all you need. You MUST calibrate BOTH in Windows as well as in AH. That is true with all axes, not just the stick.
Hope that helps.