There seems to be a significant GAP in the Credit and collections automation process. Microsoft designed the product to look into only the oldest/leading invoice per customer and then apply the collection automation process only against this single invoice. Hence the suggestion to enhance this logic so the collection automation process is run against every single outstanding invoice against each customer per LE. Based on this then you can assign an dunning letter and/or activity per outstanding invoice as long as it's overdue.

The enhancement will BE enabled/disabled via Yes/No toggle per each LE located in the Sales ledger parameters > Collection process automation > General 

Toggle = Yes - enhanced logic to include all outstanding invoice for each customer in the LE.

Toggle = No - revert to standard logic for leading/oldest invoice per customer in each LE.