Wordpress' wp-cron.php only executes on page view, thus it is not a true cron. A more reliable way to never have a missed scheduled item is to use an actual real cron job.
Here's a link to a pretty straight forward tutorial on the matter:
http://dansgalaxy.co.uk/2010/10/03/h...press-wp-cron/
On a high traffic site, you'd probably want the cron to run it every 5 or 10 minutes. Or, if you have specific times during the day when you post, set the cron to run a few minutes after each daily schedule post. You'll never see missed schedule again.