Unable to read the backup tapes

My Nt file server is connected to Hp Surestore 5000 backup device. But I was surprised when I tried to get some files from previous backups made about a year ago, the tape was detected as a foreign tape. The backups was made by the previous IT staff, before I joined the company.
What causing this? I am able to read the backups that I made but not the old ones.

My server is running on NT4.0 (sp5.0)
Backup software: Cheyenne ArchServe
Backup device : HP Surestore 5000

Please Advice.
Thankyou.