An agent that has a backup schedule limiting its backup to only run during a small window/timeframe may get queued if multiple agents are already transferring snapshots due to the maximum concurrent transfer setting. The maximum concurrent transfers setting specifies how many backups can run at the same time.
This alert does not indicate the backup failed or there is a problem with the agent itself, it is simply warning you that the agents scheduled backup was unable to start within the backup window.
Adjust the snapshot schedule so the agent has more time to start the backup you require or adjust the concurrent transfer setting so that more agents are allowed to perform a backup at the same time.
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