The reasons for failed Citi credit card swipe authentication include abnormal card status, abnormal POS machine, and the card does not support transactions.

1. Abnormal card status: Citi credit card card status is abnormal, such as the card is degaussed, the card is not activated, the password is locked after entering the wrong password too many times, the card has expired, causing the card authentication to fail. It is recommended to call the customer to confirm.

2. POS machine abnormality: The POS machine signal is not good, and it is not connected to the network, which may also cause the card authentication to fail. It is recommended to try other machines.

3. The card does not support transactions: For example, single-logo credit cards do not support UnionPay transactions, so they cannot be used in some POS machines, causing card authentication to fail.