The most common reasons for refund getting declined are:

• Refunds are not supported by the card account(certain prepaid cards, etc.)
• The card account was frozen* due to fraud
• The cardholder closed the card account
• The cardholder has already disputed the transaction

* Might be initiated by both the cardholder and the bank itself.

We recommend offering the cardholder to contact the issuer to clear the situation out.