"Shutting up shop" is a tried and tested method of seeing out games. I think our issue is that we don't really have the personnel to do it properly. Just adding another body in midfield isn't going to help unless they are a defensively minded player. Adding an additional centre half, which we have also done in games, just seems to confuse the rest of the defence as to who is responsible for what.
I do feel to some extent it is working better though, with 3 clean sheets already this season compared to 6 in the whole of last season. We didn't get our first clean sheet until September, ironically (considering their start to this season) with a 0-0 home draw with Preston.
If you look at the Championship tables every year though, those teams with low figures in the against column are often hard to find. 18 teams averaged more than a goal a game against, with Sheffield Utd, WBA and Millwall all conceding 45. Even Champions Fulham conceded 43. It's just a really difficult league for defenders.