BuddyPress.org

Compare the two pages index and single post, you are not calling the same sidebars; in single.php you appear to be calling ‘left-sidebar.php’ and this does not have the same rules applied to it as does ‘sidebar.php, and would need to if it were to function in the same manner as on the index page.

Not sure where you got ‘left-sidebar.php’ as it doesn’t appear as part of the bp-default files.