By default no invoices are generated for cases in which no billable work has been done in the selected invoicing period.  If Generate Past Due Invoices (under Settings/Tools/Reports-> Invoice/General Settings -- look for a checkbox for this setting) setting is selected, the system will generate blank invoices showing past due balance. In the invoice listings, these invoices will be shown as 0 amount invoices, but when PDF or Word format invoice is downloaded, past due amount will be shown in the total box.  Please look for these invoices under "Fully Paid & Written Off" sub tab under the Invoices tab of a case or client.