Backup Exec isn't worst product ever made, you obviously haven't taken the time to learn how the software works and may have not tried other products. The rest suck more.
What is your "media job"? Does it target that 'Archive' media set?
All jobs target a particular media set. When you move a media (tape) to a media set, the media inherits whatever overwrite protection and append period settings on the media set. Right click your 'Archive' media set and go to Properties. What are the OW and append period settings? Next, make sure the "media job" is targeting the correct media set. On the General tab in Backup Job Properties look for the Destination Media set. Finally, remember that if you have set Tools|Options|Media Overwrite to overwrite Scratch media before overwriting Recyclable media (the default), the overwrite job will look for Scratch media first. If BE cannot find any overwritable media in Scratch then it looks in the media set it is targeting for an overwritable media (ie. media where the overwrite protection has expired). If it cannot find any overwritable media there, then it prompts you "Please insert overwritable media".
An append job will look to the targeted media set for an appendable media. If BE cannot find appendable media in the targeted media set, the job turns into an overwrite job and follows the rules of an overwrite job as outlined above. In a loader if you want to be absolutely sure that a particular job is going to pull a particular tape, partition your slots, follow instructions in the following technote:
You can then target a partition as you would a device. The slot partition is a virtual device. The technote has screenshots which I think someone like you needs.
READ THE F'ING MANUAL!!! Whether you purchased BE and received a CD or you downloaded it, look for the BEAdmin manual in .pdf form OR download it from support.veritas.com
Good luck!