This seems to be an bug, if some plugin calls “apply_filter”, sharedaddy will be marked as rendered and it will not show up again.
A quick and dirty fix is to change line 402 in file wp-content/plugins/sharedaddy/sharing-service.php from:
// Only show once
if ( isset( $shared_with_posts[$post->ID] ) )
$show = false
To
// Only show once
if ( isset( $shared_with_posts[$post->ID] ) ) {
$found = strstr($text, '<div class="snap_nopreview sharing robots-nocontent">');
$show = (empty($found))?true:false;
}