On a Windows Vista, Windows 7, Windows Server 2008 or Windows Server 2008 R2 machine, a scheduled task may not run upon reboot if the machine was off at the time of the scheduled task.
This can occur if the task trigger was set to run One Time when created. It is possible to set a task to "Run as soon as possible after a scheduled start is missed". This will cause the task to re-run after a reboot if the trigger was missed. However, this does not occur if the task is set to run One Time. This behavior is by design.
You can work around this issue by setting a time and date under the Expire option of the task. This option can be reached by opening the Properties of the task, selecting the Triggers tab, and then clicking the Edit button for the trigger in question. If a date and time are set for the Expire option, the task will attempt to refire on reboot if its previous trigger time was missed.