Workflow does not trigger automatically while it's set to start when item is modified in SharePoint 2010

I have a workflow in a document library, it routes document to a destination library based on matched metadata, it is set to be triggered when an item is modified.

But when a document was uploaded with required metadata filled the workflow never started.  [I understand the document upload operation comprised of 2 actions: created and modified. when user fills in metadata and clicks "save" button it is the "modified" action]

If I "edit properties" of the uploaded document and "save" it then the WF is triggered.  I run it as a regular user not Admin.

But strangely, the same workflow is triggered in another library in the same site with the exact same workflow and WF settings, ie. start when item is modified.

Anyone has any idea why the WF is not triggered by itself?