I'm behind the enemy, hiden AND on higher ground. How is that not advantage for sneak attack?
You may have disadvantage from some source which would effectively cancel out your advantage from being hidden. By the way, being hidden is the only one of those three things that would give you advantage.
My guess is that maybe your target was outside of your normal range. That would give you disadvantage. Your sneak attack does not work if you have any disadvantage... even if you also have advantage from another source. Of course, I'm just guessing because I don't have the scenario fully in front of me.
