If only one ONU connected to a PON port cannot be automatically discovered, the possible...

When other ONUs on the same PON port can register but one ONU cannot, troubleshooting focuses on the individual ONU or its drop fiber. Valid possible causes include: ONU not powered, fiber/connector issues, password/SN mismatch, or faulty ONU hardware. Since hardware failure of that specific ONU is one of the recognized possible causes, the statement is true.