Hi,
There'are a few possibilities :
The inventory of your library is incorrect so the library can't find the volume and sets it to status unavailable
The volume isn't physically in your library
At some point in time you had a problem with the library, all the volumes that TSM tries to access during that period are marked as unavailable
Someone changed those volumes ( for a specific reason ) to status unavailable
I would suggest verifying the content of your library first and afterwards running 'audit library'. Be carefull this can take a while ( depends on what kind of library you have )
After these manipulations you can update the remaining volumes to status readwrite to make them available again