The JAL app shows plenty of Class J availability on March 26, but only at full fare. On March 19 and earlier dates, however, there are lots of discount fares available. Probably if you wait a while (a few days or a week or so, maybe?) discount seats will become available on March 26.

As for Expert Flyer: I've noticed huge discrepancies between Expert Flyer and the JAL website in the amount of availability shown, at least for domestic flights. It looks like Expert Flyer is not a reliable indicator of JAL availability, unfortunately.