Not all people buy their games digitally on the PlayStation Network. You have to account for those who shop retail physically too. With Ni no Kuni being a January release too, it's possible that most people who were going to get the game have already done so. Rather than watching the PlayStation Store sales, you'll have to keep an eye on what the NPD charts have to say. Here's one news story that's showing the game is doing well, just not exceedingly.