Yes, couldn't get that flight combinations of flights to show up on the multi-city tool, only if added as separate flights. No other connections in the US, next flight is 5 days alter.Did you try the multi-city tool? As well as straight up SIN-SFO? It might be married segments or other revenue control.
Did you have any other connections in the USA immediately after arriving? The tool will not allow more then three flights to be booked on the same day.
i.e. if you search Singapore-> San Francisco you will get flights via Taipei
But if broken up you will get the flights via Tokyo, but you can't actually book them or you get error #4002.