SQ evaluates award availability same way revenue does - on an Origin-Destination basis, so JFKDPS must have award space on both segments in order for them to give it to you. I'm surprised SIN-DPS is constrained, given that they added more flights to DPS. Ah, noted you mention award space is only available on an option that has a 14hr layover. Basically if you do a fresh search JFK-DPS on the date you want - what you see online is what the agent can offer you.