Email reminders not working

I am trying to get the email reminder working. I setup the event with volunteer timeslots setting to send a reminder 24 hours before the event and I setup in the cron.php in my drupal directory to run every 15 minutes but that didnt seem to do the trick. Am I supposed to do anything else?

I know the emailing works correctly because when I check copy me for the volunteer event it sends that email immediately.

Thanks!