This seems like a hosting issue. Perhaps another client is sending out a large amount of email at the same time your dailymail is expected to run. Restrictions, or a crash, on your host could cause your messages to not get through. Your host might also be performing maintenance or upgrades on the server around the same time. Normally, however, in the event of a crash or interruption ListMail should show some items remaining in queue..
It's really hard to say what's going on here.. Perhaps you should change your cron task time by 12 hours to see if it helps.
