Lockbox Record with NULL Invoice Number Not Assigned to Customer

We are troubleshooting lockbox matching issues and seeing the following problem:

In all cases where a valid INVOICE NUMBER is included in the overflow record (4), the customer is matched correctly using the AutoAssociate function.  In almost all cases where a valid INVOICE NUMBER is NOT included (either wrong number or no invoice number) the total receipt amount ends up as 'Unidentified'.  When comparing the parsed MICR number on the lockbox report to the customer record, it appears that we have a valid MICR number in many cases.  For some reason, it doesn't seem that the system is able identify a customer record based on a MICR number alone and thus