From the reports I've seen from the Narita turnbacks, they are getting the FLAP DRIVE message and an associated specific maintenance message due to problems with one of the rotary slat drive sensors (or sensor circuits). The slats themselves and their actuating system were not failed, but they get disabled when a valid rotary drive sensor signal is lost. The FCOM has procedures for such an event. I have not heard anything about why they are struggling to troubleshoot the identified problem sensor and circuit.