Answer:
How is it that a credit was returned for insufficient funds? The ACH system should not permit such a return reason for a credit. Assuming that it was somehow done, I can well imagine the ODFI wondering what happened. Get on the phone (after you've figured out how this error was made) and explain the problem to the ODFI, asking that the entry be reinitiated. If the ODFI needs you to talk with the Originator, do so.