Interesting - a couple troubleshooting suggestions:
1) Recreate schedule (may just be a corrupt schedule)
2) Increase time from 30 to 1hr (rule out flood)
3) Unless the 30 minutes is needed from 12 - 12:30; recreate schedule as one line - between 6:59 pm - 7: 00 AM
4) Recreate the schedule to only pull once or twice a day. (McAfee daily DAT files are generally available by 19:00 (UTC/GMT))
For what time should I schedule a daily automatic update of my system with the daily DAT files?
McAfee recommends scheduling a daily pull task within a 4-6 hour interval from the time the daily DAT files are made available to the source repository. This allows enough time for the DAT file to be replicated on all McAfee servers globally.