It is unfortunately not always possible to successfully complete a transaction. A direct debit payment can fail for a number of reasons. A transaction can be rejected until 5 working days after the direct debit date and a customer can even charge back the payment until 56 days after the collection date.
When a payment fails, you will always see the 'reason code' for it. This code will explain why the transaction was failed. It can be found on the details of a transaction.
On this page Mollie gives an overview of all reason codes and what they mean.