I have a Windows Server 2008 64 bit, primary function is a HOST running Hyper-V. We have a simple backup routine to backup VHD's from the server, using the diskshadow command. Works great on other servers and works great if we manually invoke it.
If we add it into the task scheduler (signed on as domain\administrator at the time), and keep the "run only if user logged on" on, it works fine. As soon as we change it to "run whether user is logged on or not" and run under domain\administrator, the job never runs. Then just to make life interesting, if you change back to "run only if user logged on" it no longer runs.