When a task is triggered there are several steps which take place before a Document Log will be created. If any of these steps fail, you will see "Failed" in the QMC and a Document Log will be unavailable. This answers your initial question on why you're unable to download the Document log: it doesn't exist.
Now, if you'd like to figure out why the task failed to begin with, the process is not terribly complicated. You know at what time the task triggered and what time you got the "failed" message, so just search your logs for "Error" or "Warn" during that time period. You'll find something which is either declarative or at least searchable in our Knowledgebase (link below). The most common reasons a task would fail at this point are:
1. No Engine available
2. Document Corrupt
3. Document locked by another program (usually antivirus)