Backup Exec tape drive selection

Does anyone know how BackupExec 2014 detemines which tape drive to use for a job? For instance, my tape library has 4 drives and I just recently had to replace 2 of them. Backup jobs are currently only utilizing the 2 older drives and not the new ones. I can run test jobs to these new drives and they work fine, but our scheduled jobs are not using them. Before the replacement, drive selection seemed more random. Thanks.