An EPM data load completes successfully but loads zero rows

A data load rule reports success, and the target intersection is unchanged.

Every source row was rejected by mapping and the rule still reports success. A load that maps nothing is not a load that failed — the distinction is why this looks green.
The point-of-view is not the one you are looking at. Period, scenario, or year on the rule differ from the form you are checking.
The source file's period column does not match the period being loaded. Rows outside the period are skipped silently rather than reported.
The target members exist but are not in the loadable hierarchy. A shared or dynamic-calc member cannot receive data.

The process log's row counts: read, mapped, loaded. Zero loaded with a non-zero read narrows this immediately.
Whether unmapped source values are being sent to a suspense or ignored — and which of the two the rule is configured for.
The point-of-view actually used by the run, not the one currently selected in the interface.
Whether the target members are store-type and not dynamic-calc or shared.

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