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An agent that has a backup schedule limiting the its backup to only run during a small window / timeframe may get queued if multiple agents are already transferring snapshots due to the concurrent transfer setting. The maximum concurrent transfers setting specifies how many backups can be running at the same time.
This alert does not indicate the backup failed or there is a problem with the agent itself, its simply informing you that the agents scheduled backup was unable to start within the backup window / snapshot schedule being used.
Every snapshot transfer has its own expiration time of 10 Hours, hence within this period if the transfer job didn't have the opportunity to start, they will be deactivated automatically after the expiration time.
You can adjust the snapshot schedule so the agent has more time to get the snapshot you require or you can adjust the concurrent transfer setting so that more agents are allowed to perform and transfer a backup to the core / repo.
You can adjust the schedule and the concurrent transfer setting using the KB link provided below:
Backup Schedule / Concurrent Transfer Settings - https://support.quest.com/appassure/kb/147461