As said above, the expander bolt/plug are not the issue, it is the stem bolts on the side that hold it all in place.

Is the headset new? It could be the bearings/races were not pressed in completely, so you tightened it up ok at first but then after some riding the bearings pressed home a little more, causing some slack.

So make sure the side stem bolts are loose, tighten down through the plug so it feels nice, then crank those side bolts down hard to make sure it stays in place. (note if you have a carbon steerer you need to be more careful with torque -- but if it's a metal steerer tube (even if the fork blades themselves are carbon) you should be able to clamp down those stem bolts pretty hard.