There are a number of factors and some have already been mentioned, but in general the atmosphere declines by club the further up the table you go.
Etihad was dire in 2003/08. It's been good this season but you'll always get quiet games. There is an expectancy level now for the team to be winning at home every week.
The thing i'd change is more of an effort to get the North Stand singing. Create a singing/standing block there and have both ends vocal would make a difference. Someone mentioned the kids wanting to sing, but they need adults there to start the songs.