If you encounter a failed instalment due to the patients payment method being declined, you will need to manually re-try the payment by either re-authorising the previous payment method or using/requesting a new payment method.
What to do
Click "Holds & Instalments"
Find and select the instalment plan with the declined instalment
Click "Request new card details"
Please note that you could also cancel the instalment plan and click "Add payment" to process the remaining balance using an alternative payment method.