When payments returned and error (Insufficient Funds or genuine error) that are not a terminal error these payments become eligible for re-processing. This can be done two ways. Automatically based on your "attempt settings" or merchant initiated
Lets go over these two different ways. These both apply ONLY to Unlimited Pre-Approval payers
1. Attempt Settings - Automatic Retry
You can change the conditions by which you re-attempt failed payments by setting up and managing your "re-attempt settings" inside the Account Settings. This allows you to set the frequency and number of times you attempt to charge for this payment again after the first failure. And example setup is below. 
2. Merchant Initiated - Manual
Sometimes a payer might experience some issues regarding their payment method on file, and once resolved require this to be re-processed. This can be achieved from either the Payments Page, or the specific customer page.
If an active pre-approval is present , a small box under the column 'Process" will be present on each invoice line. Once you click this, a small dialogue box will appear asking you to confirm that you wish attempt this payment again, and the selection is highlighted. 
NOTE:
Both routes are identical as above, except the customer route appears in the payments section under the customers information (Scroll to the bottom of the customers page and locate the payment under "failed payments"
A Payment that has encountered a "Fatal Dishonour" (Blocked by Bank, Stolen Card, Fraud Detected) cannot be reattempted with that payment method, and will either require a new payment method to be stored under pre-approval.