Scheduled Taskflow randomly not running

We're running 11.1.2.3.700 and have set up a Taskflow in Workspace and scheduled it to run nightly at a specific time (with start/end dates).

Sometimes the taskflow fails to run, i.e it's not even started.

There are no errors, no mention of it in 'View Taskflow Status' or the SharedServices_taskflow log. It's as if it doesn't exist.

As I say, it's random, there doesn't seem to be a pattern. One night it could run, the next night it won't run. It doesn't conflict with any other taskflow.

Sometimes we find that if we adjust the taskflow scheduled time by 1 minute, it will run ok for a while and then fail again.