It is all because the cron.php does not run. The cron.php is for the regular tasks need to be done, including sending these emails.
You can run the cron.php manual by this url: (oops, it can be run by everyone)
http://yoursite/admin/cron.php
I think we should make the cron.php run regularly, this is what i have done. Anyway, here is more detail about the cron.php:
http://docs.moodle.org/en/Cron
