Leaderboards can be set to "Reset Daily" or not at all (called Entire Event in the options). If it is set to "Reset Daily" then unless a post was from the current day, the leaderboard will not populate—a post being approved today won't cut it, we use the timestamp of when the post was sent, not when you approved it or moved it into the displayed queue.
If you want the wall to show for the entire event, you can change this under the "Edit Miscellaneous Options" group on the Design page of the builder.