I think they need to just get out there a bit more and tell people more about it, if you're willing to spend that much money on a product (as a customer) you want to know how it will perform.
People are set in their ways re. the makeup they like and will carry on buying it unless you blow their socks off. Illamasqua is a key example, if someone had told you two years ago (or even a year ago) that a new brand would come out and people would be leaving MAC for it, you'd have thought "yeah right". But look at Illa now, it has so many fans who have used MAC for ages but now find that Illa caters better for them.
Illamasqua went to the people who have the "power" and got their brand recognised that way. If you Google Illamasqua products, you will see tonnes of reviews from bloggers. The same doesn't happen with D and G. As you said, there were a few posts and then it kind of went away.
Another reason why I think it hasn't made as big a splash is because it isn't widely available if I'm correct, it's only in a few places in the UK? I don't know about other people but I want to be able to swatch products if I'm going to be spending a fair wedge on them.
For me personally, it was the ad campaign which put me off. SJ was photoshopped beyond recognition and I am always a bit dubious about that because my thinking is "if they need to photoshop her THAT much..what does that say about how good the products are?" Having said that, LV did that to Madonna and their bags stand the test of time BUT then again, they are established and people know they're good. Plus, photoshopping a bag doesn't change how sturdy it is whereas you can photoshop makeup to make it look more pigmented.
I just think that people know what they want, they know which high end brands do glamour well (Dior, Givenchy, Guerlain) and which ones perform as they wish for them to perform and that you have to work hard to get people to recognise that your brand is good. I feel that Giorgio Armani are kind of in the same boat. Everytime I have been to Selfridges, I find that their counter is quiet. I went at peak rush time (Saturday) and you wouldn't even get the MAC counter but the GA one was empty, but they must be doing something right as that counter has been there for a lonnng time and I bet ground rent in Selfridges isn't cheap!
I also think (As the other poster said) they should have really come out with one product that is a right blinder. Illamasqua have their molten liquid powdery things which people know them for and everyone loves their blushers. Brands like Chanel have recently done very well with their nail polishes (Jade, Paradoxal etc..), D and G need something like that and people will flock.