An integration reports success but the target system never receives the data

The integration instance completes without error and nothing arrives at the other end.

The target accepted the message and rejected it later. An asynchronous endpoint acknowledges receipt, not processing, so success here means delivered rather than accepted.
A mapping produced an empty payload. Sending nothing successfully is still success.
A filter or condition skipped every record. A routing rule that matches nothing completes cleanly.
It went to the wrong environment. Connection endpoints are per environment and are the easiest thing to carry over from a test.

The actual request payload sent, not the mapping preview — an empty element is visible there and nowhere else.
Whether the target has its own error queue or rejection log, and what is in it.
The endpoint URL the connection resolved to at runtime.
Whether the instance count matches the number of source records you expected.

If none of the above explains it, raise a ticket and include what you checked. Reading this article cost you no hours.