The top cap that comes with campy hiddenset threadless headsets has a thick round plastic ring fitted to their underside. this ring can interfere with proper headset adjustment if it touches the top of the steerer tube and can cause problems like what you describe. There are two primary options to deal with this:
1. Cut the steerer tube a little short than one would normally to prevent contact (not ideal)
2. Remove the ring by easily popping it off. This ring is fairly superfluous and its removal has no negative impact on performance of the part.
-j