MaxShutdownDelayMin is defaulted to a value of 10 minutes.  Inspecting Event Viewer and CDF traces captured from the DDC, you will notice the powered off machine has exceeded the shutdown timeout period: Worker(SID) failed to reboot/shutdown within allowed time

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Instructions

HKLM\SOFTWARE\Citrix\DesktopServer\RebootSchedule 
Name: MaxShutdownDelayMin 
Value Type : REG_DWORD 
Default : 10 
Info: Minutes Minimum=1 Maximum=60 
Description: Maximum time allowed for a VM to shut down during reboot cycle processing before the reboot of the VM is deemed to have failed. 

Note: Please take backup of the registry keys and snapshot of the VM. 



Citrix Cloud sites - Powershell command must be used to extend the default timeout.
Reference: https://support.citrix.com/article/CTX272494

Note :- In case of Citrix cloud - the same can be increased by running a powershell command on Citrix cloud managed DDC . 
 

Caution! Using Registry Editor incorrectly can cause serious problems that might require you to reinstall your operating system. Citrix cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. Be sure to back up the registry before you edit it.

Refer http://support.citrix.com/content/dam/supportWS/kA460000000CcAHCA0/RegistryKey_XenDesktop_7.x.pdf