SetThreadExecutionState(ES_DISPLAY_REQUIRED) does not turn the screen back on

I'm trying to write a powershell script to run a few tasks in order to test the batteries on a wide range of tablet PCs.

A big part of this is controlling the display. I don't seem to have too much trouble turning it off, but when I come to turn it back on again, it only seems to work on some machines.

I have found this very helpful code:

#Load the Windows API from a specified dll - here Kernel32.dll
$function=@' 
[DllImport("kernel32.dll", CharSet = CharSet.Auto,SetLastError = true)]
 public static extern void SetThreadExecutionState(uint esFlags);
'@

$method = Add-Type -MemberDefinition $function -name System -namespace Win32 -passThru 

#Specify the flags to use them later 
$ES_CONTINUOUS = [uint32]'0x80000000'
$ES_AWAYMODE_REQUIRED = [uint32]'0x00000040'
$ES_DISPLAY_REQUIRED = [uint32]'0x00000002'
$ES_SYSTEM_REQUIRED = [uint32]'0x00000001'

$method::SetThreadExecutionState($ES_DISPLAY_REQUIRED)

This will work fine on about 60% of the machines I try, but not the rest...

Is there something I'm missing?

Thanks in advance