Start with the obvious: All savvy people run adblocks. So to hit the whole market is impossible. I'd never see a GW ad if they placed it.
Next: There is no point advertising GW to wargamers. We know it exists. So that venue is missing.
So where DO you advertise, given it costs a lot? I think a TV advert would be an idea. So long as its not... terrible. (Example.)
As has been mentioned so many times before, GW like using their stores to advertise. In the UK they are in city centres next to well known clothes shops, food stores, etc. so that "mum" will trip over them. They stick them in shopping centres at high cost, to advertise.
Its possible that they then don't want to spend even more on advertising when so many adverts go ignored.