Thats just the way award availability varies. Its not just an SQ issue. Unfortunately you usually have to be pretty flexible with dates in order to get the route/class/carrier you want.

I find that SQ and LH for that matter usually open up unsold C/F seats about a week before departure

If you tell me the date/route/class your looking for, i can tell you what availability is like on that date, and therefore you can work out whether it is likely they will release seats for rewards