SQ marches to its own drummer regarding release of award seats to DL. There doesn't seem to be a discernable pattern. The only consistent thing is that when you get offered a seat, snare it quick.

I took my sister to Germany via SQ JFK-FRA in J. I could only get one seat on each flight so we made plans to travel on consecutive days and meet up in Frankfurt. However, I continued to call DL, even after ticketing, to see if SQ had released any more seats. At about 5 days from travel, SQ released another J seat. I was happy to pay the $50 change fee to get us both on the same flight.

Moral of the story - take what you can get initially, then keep trying up until your date of travel.