Bob,
Let me throw another reason out here - from personal experience. I think you touched on it indirectly.
The main reason I've found for backups simply not starting - AT ALL - is that the service account I use for the DP services is not properly in the Administrators group in DP. Once I fix that, things are fine.
The only question I have on this is that NOTHING shows up in the logs on the session that didn't start. This caused some grief here which led to an upgrade of Storage Essentials to try to grab reports on DP sessions that do not start. We'll be doing some testing on that today.
Ken