There is a nice doc from Microsoft regarding Idle Conditions: https://msdn.microsoft.com/en-us/library/windows/desktop/aa383561(v=vs.85).aspx
Useful to understand why you schedule is not running!
Creating a Task, on Conditions Tab you will find these options:
“Start the task only if the computer is idle for”: This means that the “Idle Condition” should me true for the time specified.
“Wait for Idle for”: If the idle timer (first option) is not met when the task is first triggered, the task will wait for the time specified before aborting the attempt.
On example this means the task will not start if the computer is not idle for at least one minute within an hour of the scheduled trigger.