Good luck. As evinced by the varying experiences in this thread, there does not seem to be any magic formula to get SQ to give the discount. The things I'd suggest are asking the agent to do a manual booking, check with a supervisor, refer to the award chart with includes F awards even from countries where SQ does not fly in F, etc. Above all, perhaps get the agent to hold the reservation for you while you consider your options and/or deposit more miles in your account. Then try another agent later...and another later...etc.

Hope it works out!