Email Subject tag for Send notification will randomly return blank value.

I have a simple notification workflow that runs on a document library when an email is received by the document library.  The send notification step responds back to the creator of the document (the person who emailed the document) with the subject of the email and the time/date the document was received.  For some reason, the workflow will capture the subject tag fine on the first time of the day or whenever we re-publish the workflow.  Anytime after that even if it is different documents, the subject tag is blank.  Any thoughts as to why this happening?