TIH 2010 Sceduled task Logon Failure

I am running TIH 2010. For some tasks only, and intermittently, I get the task failure due to a logon problem with credentials. I am administrator and the PC has no passwords.
So why would I get these logon failures?
If I go to 'Tasks and Logs', and manually start the task it runs with no issues. Only at the scheduled time do some fail.
Please don't advise to delete and recreate as this is often a newly created task that has the problem. Why does it not work reliably?