Some transactions may have a FAILURE or NOT_ACCEPTED status :
1 - "FAILURE" status :
This transaction status appears when the bank chooses to reject the transaction. In the "Advanced" tab, you can find out the reason given by the bank with the "Bank Code".
In the case of the bank code is "A1 - Repli VADS" and the bank message "Soft Decline", the bank has declined your transaction due to a lack of securisation. You can retry your transaction with 3DS to settle this problem.
More information on the bank codes here.
2 - "NOT_ACCEPTED" status :
This transaction status appears when an acceptance rule is not respected by the transaction. In the "Advanced" tab, you can see the acceptance rule that has blocked the transaction.
To find out which acceptance rules your transactions are subject to, go to "Security" then "Acceptances" :
More details on the acceptance rules here.