It can be sporadic, though they do seem to be relatively consistent about releasing some seats close to departure ...
That being said, if the flights are sold out, then there's not much that you can do. I just checked Expert Flyer, in early May many of the flights are either sold out or close to being sold out.
AUH-NRT
2nd May - Many J seats
3rd May - 7 J seats
4th May - 3 J seats
5th May - 0 J seats
6th May - 0 J seats
7th May - 3 J seats
8th May - 3 J seats
9th May - Many J seats
10th May - 2 J seats
11th - 13th May - Many J seats
14th May - 5 J seats