I've bene scratching my head about this for some time now. I'm using the Steam 4C2 plugin to integrate Steam functionality, and everything works except leaderboards. Achievemtns unlock, it connects, it even finds the leaderboard, it just won't upload anything to it.
I've attached a file of what I'm doing. Also can be seen here: robbotto.com/leaderboard.png
Score being a global value.
Like I said, it can find the leaderboards, and I'm running through Steam, and I've double-checked the leaderboard names etc.
Has anyone used leaderboards with Steam4C2? I've emailed the author, but haven't gotten a response.