I have had similar problems, typically it is waiting for a tape (even though you have put one in) . The window for the backup to start in must be wide, once the window has expired the job will go from queued to missed. Try this:
1. Run a catalogue job 1 hour prior to the backup if you have a tape library.
2. If you are using media sets (daily/weekly/monthly) then in the media manager set the overwrite and append period to 0 so that the media can be overwritten immediately. The daily set defaults to 7 days protection. If you label new media, even though you have not written to it, it prtects it for the overwrite period.
3. If this gets it working then you can fiddle with the media sets until you get it right for you.