chat message delivered but not archived (written to the database)?

We built a GUI front end for the chat service. We have Openfire 3.9.1, monitoring service 1.4.0. We are using MS SQL.

Recently one of our user noticed that his chat conversation was delivered but one message was not archived.

We didn't locate any database error message. Unfortunately the error log for that critical time period was rolled over so we cannot verify if there's an error message for that.

My questions are:

1. What could be the cause of it?
2. Could there be an error log somewhere?
3. How can we verify it and either log an error or prevent it from happening?

Thanks

Jules