Flaky execution of scheduled backups? No Sat. AM in "Daily"?

[DrDan in MA]

I've been having difficulties making SD! do backups regularly. I'm running SD 77 on a PBG4 and a Mini, each MacOSX 10.4.5 (all updated) and each with its own external, mounted, formatted FW HD with a previous Full Backup already present on that partition. I've got both systems set to wake up at 0320 each day, and a smart backup set to happen at 0330. Sometimes it works, but most mornings when I wake up, the backup has not happened! Howcumdat?

What more info can I/should I have supplied to make this question answerable?

OBTW, it appeared one Friday evening, when I scheduled a backup for DAILY at 0330, that the next scheduled backup was SUN at 0330. However, as I'd hoped, when I awoke on Saturday AM, the 0330 backup had happened. So this may be simply some feedback problem...