If I put a Desktop Delivery Group for XenApp servers delivering service desktops into maintenance mode, then I shut them down, I do maintenance, then take the delivery group out of maintenance mode, none of the XenApp servers start on their own. I want the whole pool to start. I have tried to create a Power Time Scheme for the multisession Desktop Group, but Powershell will not allow it and asks to use Reboot Schedules.

Explained below, that by design the Server VDAs do not have power management schedules like the Desktop VDAs do. So when the machines are down and maintenance mode is removed, the Server VDA's will not boot up like the Desktop VDA's. 

Determination:
After reproducing in my lab the same outcome occurs. When I shutdown the Server VDAs (I created 3 for testing), turned on Maintenance Mode, then turned off Maintenance Mode the server's do not turn back on automatically. This feature is not included in Server VDA's only for the Desktop VDA's. 

Please see the following article: Citrix Documentation - Machines. 

An Excerpt from the article: 

"Power manage machines 

Note: You can power manage only virtual Desktop OS machines, not physical ones (including Remote PC Access machines). Desktop OS machines with GPU capabilities cannot be suspended, so power off operations fail. For Server OS machines, see Create a restart schedule" 

Note how it states that you can only Power Manage Desktop VDAs, for Server OS VDAs see the "Create a restart schedule". This power management section explains how to set the Pools and Buffers (How we tried setting the Peak and OffPeak buffers that are meant for Desktop VDAs), Power state timers, and power management schedule. 

And if you scroll down the article to "Create a Restart Schedule": 

"Note: You can use a restart schedule for Server OS machines only. For Desktop OS machines, see Power manage machines." 

Restart schedules are only for Server OS VDA's. 

This explains how it is currently not possible to perform power management schedules for Server VDA's.

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Problem Cause

When customer would remove maintenance mode from a group of Server VDAs they would not turn back on like Desktop VDA's.

This article explains the following: Customer wanted to know why his Delivery Group with 20 Server VDA's (Windows 2008 R2 Servers) when placed in Maintenance Mode and shut down why they would not automatically boot up like Desktop VDA's do when Maintenance Mode was turned off. This by design, as Server VDA's do not have power management schedules like Desktop VDA's

Citrix Documentation - Machines