This may be an issue for me mainly because I have always lived in Tokyo, but I have never had good experiences with Japanese laundry powder. There are several problems that I have with it. The first is that it doesn't dissolve in the cold water that you have no choice but to wash with. The second is that it always draws damp before I can use even a small box up. Invariably, I am left with large, rock-like clumps that are even harder to dissolve than the powder and I just have to toss the rest out. Third, for a small quantity, it's a little expensive (particularly considering that about 1/4 of it is wasted). I converted to imported liquid laundry soap and never looked back, but occasionally still get free boxes or samples as part of various promotions.
Perhaps there is some special technique for dissolving it or a way of storage for Japanese laundry soap powder (or my washing machine is somehow not up to an important task), but I won't miss the hassles associated with using it.