I have 20/21 as well and it has been driving me nuts, lol. I have 21/21 in Emerald graves, and that was a pain in the buttox as well. There was one really tiny region that is very easy to miss, which I thought might be what was happening with the plains too, but I had no luck with that. I am also missing one region in Emprise Du Lion.
i cant find the 21st region as well.
It's a pain but in the south end of hte elven graveyard there is a una'din grotto. I wasn't going grave robbing on my play through so missed it till I was region hunting. You have to run right up to the door to trigger the region, but you don't have to do the grave robbing quest.
