Zardnaar
Legend
There's another reason for slightly softer sales on recent releases as well: Many of them have not reviewed particularly well (Spelljammer probably being the worst) and are a bit of a departure from the norm (like Tales from the Golden Vault). Phandelver is more "normal" AND is doing pretty well (but it's early yet for that project, AND it's 1/3 reprint, which probably negatively effects it, even if it's reprint of something much loved). This and the increase to MSRP, which effects sales negatively when it's first introduced, but will eventually "feel" normal as time goes on.
Hopefully Planescape will review well.
What's the deal with Golden Vault? It's in shrink wrap at mah store.