As a B&N employee (and the guy who orders all the D&D stuff at our store), I can tell you for a FACT that the amount of D&D (and RPG stuff) at a store is almost totally determined by whether or not an informed and interested person is working at that store.
I have ordered every single D&D thing since 4e came out (and before), and it steadily increased our sales. However, even now, I have to increase all of the initial orders of this material, or we only get 2-3 copies of each new book as it comes out.
This is largely because our buyers do purchasing for entire regions of the country, and often don't have the time to look at the sales history of a particular store. Our RPG buyer is also our SciFi buyer, and while he is a GREAT guy, he's making purchasing decisions for every B&N in the country for all SciFi/Fantasy/Graphic Novel/Manga and RPG titles. No wonder he can't devote much time to it!
In addition, he has no real way to gauge whether an area has a FLGS which will capture most of the sales, or whether that B&N is the source for gamers in the area. He also doesn't know whether a particular D&D book has good buzz, or has been previewed to rather lackluster response on WOTC's site.
The local store has to have a gamer who will do that stuff, and do it off the clock. If I wasn't into playing D&D, I wouldn't know nearly as much. As I don't play much of other RPGs, I'm sure they get under-represented at our store too. I'd like to do more, but that research would basically be volunteer work.
One thing I can tell you that will help, if you want your local B&N to stock more D&D, is not just to buy there (of course that helps). Pre-order your books there if you plan on buying them there. If you choose to 'pre-pay' for the book (and have it shipped to you directly, which can be free depending on how much you buy), you can even get the online price. If you don't see something on the shelf, ask them to order it. The amount of D&D a store gets is influenced off their sales, of course, but also off their preorders.
Waaaay too many times, someone comes in looking for a book and just looks, doesn't see it, and leaves. Even if you don't want to order the book, ask an employee about it. Many of us will order books for the store, based on customer requests, even if the customer doesn't order one for themselves.
We can't read minds. If I don't play Pathfinder, how am I gonna know that a new supplement is coming out? How am I going to know that ten people want to buy it from us on the day of release, and not two?
I'm not trying to drum up sales for B&N on this. The same thing applies to your FLGS, or even your local book store. It's always going to help if someone at the store plays "your game". However, even if no one does, you can often get your local store (whatever it may be) to carry more stock of stuff you want, if you will talk to them.
On the D&D minis, I can tell you one thing. The newer boxes were a HUGE improvement over the old card boxes. It was a real shame that they rolled those out basically as they were ending the miniatures game. The older card boxes were a huge theft problem for us. People would open them, and either take all the minis, or re-distribute the minis they wanted, and then buy those boxes. Most stores in our area gave up on carrying them due to theft. I moved all of ours behind the counter, and put empty "display" boxes on the sales floor, so you could pick out the expansion you wanted, take the box to the register, and buy "full" boxes there. Not many B&Ns are willing to take the time to do that for a relatively low cost item (compared to the computer books and medical textbooks that are also being stolen). The newer boxes were much more resistant to those theft shenanigans.
