It might happen that at times a merchant's payment gateway does not accept payments from certain cards including SlicePay.
If a specific transaction failed then please share below details to help you better
1) Merchant (where you tried to use card)
2) Transaction Amount
3) Date of transaction
In cases of transaction failure, the amount is debited from your account then it is instantly refunded. In case you have not received the refund then please visit the "I have a refund related issue." section and raise a request with the required details and we will assist you further.