Error: Manual matching required. Could not determine a membership.

I'm getting this error in the billing transaction logs, but not receiving an email when this failure occurs: "M1 Payment detected. Manual matching required. Could not determine a membership."

First, it would be great if it wouldn't fail (what's causing the matching error?) and secondly, I really want to know about it so I can manually fix it. Is there a setting that I need to adjust to receive an email when this happens?