The top cap bolt in the pic might be two separate fasteners. You can see "and were asking about" how the bolt turns everything. Try to undo the bottom larger diameter threads from the top allen head bolt. The larger bolt may have its own Allen head and is hollow with inner threads which is used to set the plug solidly in the steerer on it's own so that the smaller top bolt can have something to anchor to when setting preload. You may just not have been able to put enough torque on the top bolt due to the slipping expansion plug in the steerer. I'm not positive that this is how your setup works but possibly.