The report is not showing all debtors in the system.
This report is working based on the last sale date of the transactions on the debtor record. And it is also only looking at transactions with a subtype < 10, meaning only debit transactions.
So if the debtor has no debit transactions within the period range specified and only credit transactions then it will not appear on the report.
Add a criteria for All, Active and Inactive and that criteria is all that it looks at to work out what to report.