I'm not that worried for this sale numbers. They are OK. Season 1 was a curiosity so almost every fan wanted to buy day one. Now that it is something normal these releases I think there is no urgency to buy day 1, so the 2-3 month period is much better indicator of the success than the day 1/week 1 sale figures.
I think the more concerning thing would be that they're difficult to find at local retailers on release day now. I went to a couple different Wal-Mart's looking for season four and they hadn't even received any after carrying seasons one, two and three. Plus people around here were having issues finding it at Target, my local one had ordered something like four copies for release day and I haven't seen it in stock since.
I can't speak for how they're selling but I did notice it was tougher to find season four on retailers shelves (in my area).