I would try Recycle = yes in pool definition or set one of the purged volumes 
status to recycle and see if it will mount then.

Dave

Pool {
  Name = Pool4
  Pool Type = Backup
  Recycle = no                       # Bacula can automatically recycle Volumes
  AutoPrune = no                     # Prune expired volumes
  Volume Retention = 40 days
  Maximum Volumes = 100               # Limit number of Volumes in Pool
}
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