You have to USE the ability to sneak attack, it's on the hotbar on the bottom right of the screen. It won't trigger on its own. If you meet the criterias for sneak attack, then you will get extra damage.
If you bothered to actually read the post and look at the pictures, or even read the responses on this thread, you will see that I did USE the sneak attack ability, as it is listed in the combat log. Someone else already pointed this out on this thread, too.
