According to their records, which I requested they send me, they sent 4/5 of my NHK order on the 14th and one of the 12th. Two arrived on the 17th, another two then a arrived something like three days later. Vol. 4 never turned up.
Going on the dates all 5 were supposed to be sent, the dates they arrived and the unavailability of Vol. 4, it's my assumption that their records aren't being updated correctly. Since they take payment when you order it's up to them to spot the error, and on multiple item orders they prbably send part and don't realize that they missed part of the order.
Of course, the unavailability of Vol. 4 and it not arriving could be a coincedence - it could've just gone missing. But in the years I've spent at my address, only a few things have ever gone missing...