Suspend after Timeout Not Happening

I'm having a problem with the system going into suspend.
1. When I press the power button it goes into suspend fine.
2. When I use the menu Start/Suspend it works fine
3. When I wait for the timer to go off (i.e. 5 minutes) sometimes it will go
into suspend but usually it won't. I haven't been able to figure out the
pattern.

Is is possible that a driver is stopping the system from suspending? If so
what is the mechanism that a driver would use so that I can find the culprit
and perhaps modify it?

I've set the registry settings under
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Power\Timeouts
s

Is it documented anywhere what
[HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Power\State\Suspend\Flags] mean?

Thanks

Ian

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