The volumes are in a couple of places.
LIBVOLUMES Table
DRMEDIA Table
VOLUMES Table
VOLHISTORY Table

It is a long explanation of how these tables are intertwined.
First check to see if you have a limit on the time of database backups by
doing a query DRMSTATUS.
Second, look at volumes in the DRMEDIA table using q DRMEDIA or select
commands.
Third, are you purging volhistory information?
Fourth, are you performing checkins of database backup tapes so they will
get scratched and become available.
Fifth, an audit may not be a bad thing to do after you learn how to use it.
Sixth, are you running reclamation on your offsite pools.

You say lost, what do you mean?

What you want to do is look in the DRMEDIA table and LIBVOLUMES tables.
Database backups and Copy Pool volumes are put in the DRMEDIA table as they
are created.  When the tapes return you have to do a move drmedia
tostate=onsiteretrieve for the tapes so they will scratch.

I use to have a select statement that would show the inconsistencies between
DRMEDIA and LIBVOLUMES so that I could perform MOVE DRMEDIA ONSITERETRIEVE
commands for tapes that had been checked in but not status changed.  You can
run a select or query drmedia and create these commands.

What you are interested in is getting a complete inventory and figuring out
what the actual piece is that you have not been doing that has lost so many
volumes.

The VOLUMES table has the information for tapes that are in primary and copy
pools but not db backups.  VOLHISTORY controls this.

DRMEDIA is an extension of these two tables to help automate the DRM
process.

LIBVOLUMES is an inventory of what is in the library.

Essentially, you should be able to account for everything between the
DRMEDIA table and the LIBVOLUMES table.

I think what you are saying is the tapes are not in the LIBVOLUMES table.
They will not be until you do a CHECKIN command of the returned tapes to
STATUS=PRIVATE.  If they are empty they will change to scratch.

The other possibility here is that you have a lot of database backups that
you do not need.  The volhistory table controls them from being scratched.
They do not show in the volume table.

Another possibility is that you are not running reclamation on the offsite
pools and they are not returning.

Are you using the PREPARE function?