I'm sure you've probably already checked this, but there are 7 or 8 services for BE8.0 that need to be running for the entire system to function as a "Service".
With BE7.x there was an extra service called "VNCSNAPSHOT" (for OFO) that also needed checked. That one is easy to miss because it is in an entirely different place in the services monitor.
You may want to try the following steps:
1 - manually stop all services then manually restart them.
2 - Schedule a small test job for a few minutes into the future and place it on hold.
3 - Make sure the backup job is set to eject the tape after it is finished (much easier to track its completion when not running the GUI).
4 - Mount an overwritable tape in the drive.
5 - Take the test job off hold and close the BE GUI but don't log out of NT.
Keep an eye on the system clock and you should see the tape get initialized within 60 seconds of the clock turning over to the scheduled launch time. After the job is finished, it should automatically eject the media without the need for the GUI to be running.
Once the tape is ejected, you can re-launch the GUI. The Job should still be running until you respond <OK> to the "remove media" prompt.
Having said all of that, I've never had trouble with any version running scheduled jobs while the application was not running live. Normally, I don't bother to logoff of the server when I'm finished. I merely <CNTL+ALT+DEL> and choose "Lock". When I arrive in the AM, I just unlock the server and launch the EXE.
I hope this helps...