Are you trying to book a stopover flight by any chance? I ran into the same issue trying to book SQ24 with advantage business rates, but with a stopover first from BKK->SIN. Would see availability but get immediately waitlisted every time and the customer service reps had no idea what was going on.

I managed to book SQ24 by itself without any other stopover legs though, so maybe SQ just blocks certain stopovers from happening without any clear warning.