JAL awards in general are harder to find since Japan has reopened, not just through Alaska. Have you looked at JAL availability through other partners? It's the same. The reasons are many; Japan has re-opened so there is more demand, Japan has not reached pre-Pandemic numbers so there are fewer flights, JAL would rather sell a seat for cash than as an award so they may be holding back awards until they get a better handle on what the new supply/demand situation will be.

East Coast is much more difficult. You might have to consider another gateway. That said, BOS-NRT has a few days at the end of schedule with availability - mostly in economy. Are you looking for economy? Booking within the last 5-6 days of schedule will require either the Calendar Trick (not 100%) or the Round Trip workaround (Reverse Round Trip if you need a one-way).

I agree with WillTravel4Food, the very end of schedule (330-331) days out is the best time to try to book something, although you should be pre-checking availability on other partner sites beyond 330 days to get a feel for your chances. If you don't find anything then you should keep looking every day until you find something because windows of opportunity will usually pop up eventually but they will not last long so you need to be ready to move quickly when something becomes available.

Beginning to mid April is peak time for North America to Japan so your chances are less that by the time the AS booking window opens that there will be something leftover that has not been poached already by other partner programs that can book beyond 330 days. And, if there are seats still available in peak times at 330 days they will go like hotcakes. Book immediately if you find what you want, because others will be doing exactly that.

Did you ever play Musical Chairs when you were a kid? The opening of the 330 day window can be like that. The opening of the window is when the music stops playing and everybody jumps on any available chair and there are always kids left standing without a chair.