There may be many reasons, which can be summarized as follows:

First, there is nothing wrong with the card. If the signal of POS machine is not good, the off-line transaction will fail.

Second, if you hold an IC chip credit card, because UnionPay has turned off the downgrade transaction function, if the cashier still operates the transaction by swiping the card instead of inserting the card, it will not be available.

Third, the card does not support transactions. For example, VISA /JCB single logo cards do not support UnionPay transactions or some online transactions, but support CCB ATM withdrawals.