If the refund button is still showing after processing a refund this means that not all parts of the transaction have been refunded. You will see the partial error when part of a transaction has already been refunded or one or more of the payments you made on a sale cannot be refunded at this time. The refund button will be available until all parts of the refund have been processed, or when 180 days has passed since the date of sale.
You can check the status of all payments on the transaction page.
Once all parts of the transaction are refunded the refund button will no longer be available.