I'd suspect your problem is with your overwrite protection settings within your media set. The quick way to check if the media is still protected is to inventory the tape after you've inserted it in the Devices section, and then selecting HP 1 in the left hand pane, check the colour of the tape in the right hand pane. If the tape is displayed as blue it's available to write, if it's black then it still has protection on it, and you won't be able to backup to the tape.
If you go into the media section, go into Media Sets, and check the properties of your media set (probably Media Set 1 unless you've created your own one), and see how the settings compare to your tape rotation period. You want to have enough time in the overwrite period to protect the tape if it gets put in at the wrong time, but not too much that the next time it's inserted it's still protected.
If you're not sure, post what your settings currently are along with your media rotation period, and we can suggest where it might need amending.
Keith
