Is this supposed to be for a round-trip or open-jaw? Is this a stopover trip or are you just connecting in NRT?
Using July 5 and July 12 as sample dates, I was able to pull up open-jaw and round-trips (all roundtrips were either SIN-NRT-LAX or SIN-NRT-DLhub-LAX and vice versa), so it appears they either fixed it or it was a temporary bug.
It's a temporary bug which pops up every now and then for me. I'm getting SIN-NRT-LAX return now, but for SGD $4,800, vs $2,200 on Zuji/Travelocity