I am going to focus specifically on compliance reasons. You can always guess, but the bank is not obligated to provide you with a reason (that’s part of compliance guidelines; and especially if your transaction falls of AML).

In almost all cases, the real reason your transfer is denied has to do with AML – but no (properly trained) bank staff will tell you that.

Beyond AML, there are two other reasons:

1. Bank’s internal polices – for example, most Islamic banks do not allow transfers to/from casinos, bars/club, or other gambling locations. In fact, for most Islamic banks, their cards also do not work at these establishments.
2. Bank doesn’t deal with the destination country – if there is a large risk of fraud or AML/TF the bank may choose not to do business in that country.

These are causes for rejection, in cases where there is information missing, most banks will contact you, For example, they might ask for the full name of the beneficiary, or the purpose of your transfer, or details of the bank. In case the transfer was rejected by the correspondence bank; most banks will also tell you why (keep in mind, AML flagged transactions are almost never actually dispatched from the source bank).

For most other administrative reasons (such as the wrong IBAN or SWIFT/FEDWIRE code was entered) the bank will contact you and resend the transaction.