Salam,
     I think that this problem happens when there are no more scratch tapes in 
your Storage Pool. There may be alot of scratch tapes in the
library but only a particular number of them has been assigned to the storage 
pool to which your node is defined.

To do a quick check:     do a q stg storage_poolname f=d and look for the parm 
maximum number of scratch tapes and then
do a q vol stg=storage_poolname and count the tapes in the pool.  If the limit 
has been reached then increase the maximimum number of
scratch limit in the pool by issuing update stg storage_poolname maxscr=xxx 
:where xxx is the number of tapes allowed in the pool.

Hope this helps.

Mansoor