This may help, I'm not sure but this was my problem many times until I found this out:
You must group your media in backup exec.
This is called a media set.
1. On the navigation bar, click Media.
2. Under Media Set Tasks in the task pane, click New Media Set.
3. Type in a name you can remember and recognize easily.
4. Drag all the tapes you are using, into this Media Set.
5. Set the media set to:
* Overwrite protection period = Infinite - Don’t Allow Overwrite
* Append period = Infinite - Allow Append
++ And now the most important trick! ++
Next time your going to do a backup, SELECT THE CORRECT MEDIA SET.
If you do not select the correct media set before starting backup, your half filled tapes are not accepted!
When you prepare for backup, you'll see the "device and Media" tab.
You'll see "when this job begins" with choices like "overwrite" and two options for "append".
Look a bit up, you'll see: "Media set:"
SELECT THE NAME YOU GAVE YOUR SET FROM THE PULLDOWN FIELD.
If you select the name your media set has, all half filled tapes will be accepted and filled until they are full.