Are you sure? Is the top cap resting on the steerer tube or the stem? The stem should be higher then the steerer tube for the 'preloader thingy' to work. Was the preload set before them stem was clamped tight to the steerer tube. It can be set as tight as can be, but the preload has to be set before the stem is clamped to the steerer tube.

I'd begin by squeezing the front brake closed and rocking the bike back and forth a bit. Watch and FEEL for any movement, and where it is occurring. If that doesn't help then start taking things apart.

Good Luck