Did you label tapes and place them in the target media sets?
If so, they will not be Overwriteable until the OPP has expired, since when you moved them into the media set, they immediately took on all porperties of the media set as of that time (OPP and APP included)
Until you have cycled through all your tapes one time, run an inventory, and any tapes the are not displayed in BLUE will need to be re-labeled or moved to the scratch media set before use. BackupExec will move them to the Target media set(s) when the job runs.
If you are using an autoloader, and want to prevent BackupExec from using a daily tape for a weekly job, you can partition the loader, and edit your jobs so that they point to the appropriate partition rather than "All Pools"