Why were several invoices created even though the customer had “collective invoice”?

This usually happens…

1. … if several shipments with different payment terms have been selected for billing. The program then puts all shipments with the same payment terms together in one invoice. An additional invoice is created for all shipments with a different payment term, etc.
   It is not possible to invoice two or more shipments with different payment terms in one invoice.
2. … if at the customer the weekly or monthly invoice is checked. That means the program divides the shipments by week/month and then makes an invoice per week/month. For example, two shipments are from one week and one shipment from another week. So then two invoices are  created.