I never watch telly live ? record it all ? so I can't comment as I never see trailers. I did see the new series trailed in a piece in the Guardian Guide, but didn't get the impression it got much press elsewhere. Maybe a few interviews with cast/writers would have raised awareness, but I guess the press didn't pick it up.
Maybe ratings dropped partly because it wasn't as 'sketchy' as the first series? Yes, there were still great set-pieces and loads of humour, but the characters developed and there was a proper, long story arc and lots of plot.
Can't believe I'm saying that as if it's a bad thing ? but you know what I mean! Maybe it confounded expectations.