I don't think you understand how that compression plug works. I failed to explain it above so I'm going to recommend that you take it out of the fork and study it to see what's going on. It is different than a standard top cap and star nut system. The top cap on your compression plug acts sort of like a standard top cap and bolt together and the expanding wedge is like the star nut. The center bolt in the expanding wedge is only used to anchor the wedge though. The top cap has a separate hex feature (larger than the wedge's hex bolt) that you can use to turn the top cap and draw the anchored wedge towards the cap (just like turning the bolt on a standard system would draw the star nut towards the top cap).

For this system to work, you need to have the expanding wedge unthreaded some amount from the top cap before insertion and anchoring the wedge though. As shown in the picture, the wedge is fully threaded into the top cap. If yours looks like that when you are installing it, that's your problem. You need to unthread it by a few millimeters at least to leave yourself room for adjustment.

So, starting with the wedge only partially threaded into the top cap and the wedge expanded to a slightly smaller diameter than the ID of the steerer tube, insert the compression plug. Using a 5mm hex key, tighten the wedge in the steerer tube. Now, using a 6mm hex key, turn the top cap to add preload. [Note that your hex key sizes may be different but the one to tighten the wedge will be smaller than the one to turn the top cap.]

Am I making any sense?