I am in the thick of the AD Rep grind and this happened to a number of people in my guild so I post a workaround here.
While grinding rep for Argent Dawn, you must have your trinket active in the character trinket slot. You will notice the Argent Dawn Commission displayed in your buff area.
If I die and run back to my corpse to rez, I noticed that the buff did not appear in my bar though the trinket was in the slot. If you do not see the Commission buff in your bar, you will not get scourgestones to drop. And if you are grouped and a stone drops, you will not be able to pick it up.
This worked for me and others in my guild last night:
Swap the trinket into the other trinket slot and then swap it back. NOTE: Just swapping it once does not work. You must swap the trinket to the other slot and then swap it back. I also noticed that just swapping it to inventory (bag slot) and back into the trinket slot did not always work.
I reported this and Blizz suggested changing the name of my WTF and Interface folders which did not work.
Good luck!