As the rifle gets used more, the hand guards will tend to rotate more.

The hand guard end cap is retained on the barrel by the step lip for the front sight base on the barrel (trapped in between).  The gas tube is the only thing that is preventing the end cap from rotating around on the barrel.  The spines on the barrel nut are the only things keeping the hand guards from rotating on that end.  Over time the end cap will press against gas tube sides , peening the thin metal, and allow that end to slightly rotate from side to side. On the barrel nut hand guard channel, over time it gets worn and will slip on the spines of the barrel nut.  

On a positive note, slightly rotating hand guards are only an aesthetic perceptions glitch to the shooter, and hinders the actual performance of the rifle in no way.

As for a fix, even if you were to permanently attach the end cap to the FSB with a screw, the delta side of the guards will still wear from being removed and replaced, which allows twisting at that end anyways.  

We could give you ideas/ways to go buck wild for a temporary solution, but sooner or later; you would be at square one.