If it's just a single recurring donation then it's possible there's a problem with the payment for that donor. From within iATS you can find the Customer and see the past charges.
i) Login to your iats payments customer portal
ii) select Customer Database
iii) Search for the recurring donor by name
iv) Click the edit icon for the donor
v) You can scroll to the bottom and view the last 10 transactions to see if transactions were rejected.