One reason , I can think of
If the catalog ran successfully with a byte count then it's definately reading the media . Only thing you can check is that there is a check box "Beginning backup date' in restore window . It will not show any data prior to that date . Since the tapes are old , you probably have that option selected and date specified . Try unchecking that option and see if that helps .
If in case , catalog runs successfully but does not give any byte count , Try moving it to the retired media , delete it and restart the services , Do an Inventory and run catalog again
Let us know if the issue gets resolved and update the status of the thread accordingly.
Thanks
Zaki Khan