When Delivery Group scheduled time for reboot hits some VDAs reboot normally but others remain in Shutdown state until they are manually turned on.

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Create following registry keys on controllers :

Registry Path : HKLM\Software\Citrix\DesktopServer\SiteServices
Name : MaxShutdownTimeSecs  
Type : REG_DWORD
Data : 1800 (Decimal)

Default : 20 (sec)

Registry Path : ​​​​​​​HKLM\SOFTWARE\Citrix\DesktopServer\RebootSchedule
Name : MaxShutdownDelayMin  
Type : REG_DWORD
Data : 60 (Decimal)

Default : 10
Info: Minutes Minimum=1 Maximum=60

​​​​​​​Registry Path : ​​​​​​​HKLM\SOFTWARE\Citrix\DesktopServer\RebootSchedule
Name : ShutdownTimeoutRecovery  
Type : REG_DWORD
Data : 1 (Decimal)

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Problem Cause

The duration it is taking to shutdown the VDA is exceeding timeout value.
i.e. On "initiate Shutdown" from DDC the Hypervisor gets the "Turn Off VM" command.
If it takes longer than the default timeout, it can never send a turn on command for that VDA.