This happened to me recently. I had been watching to see NC open on a flight back to the West Coast from NRT. Up until T-24 hours prior, .bomb showed no NC on any flights but all sorts of open economy seats and 20+ on the seat map on the NRT-SEA flight. At the 24 hour mark, . bomb showed NC4 the NRT-SEA but ALL economy seats were zero'ed out. I know this because I was online and the phone with United Japan when the issue occurred. The phone agent in Japan was able to work something out after 20 minutes, switching me from my original NRT-SFO flight to NRT-SEA and upgrading me into C. I believe there is a software bug in the system that sometimes negligently zeros out economy availability for a period of time. The flight went out about 65-75% full in economy and C had five open seats.