bdchamp, Dav also worked with young bunch of players and he had to go through a lot more chopping and changing in search for a reliable a nucleus of the team. He finally found the core in the last year of his tenure and that's why he succeeded regardless of opposition.
Dav introduced Tamim and Mushfiq to international cricket who played match winning innings in the world cup 2007.
Siddons received a more mature team and better players like Rakibul. Still he could not produce better results in batting let alone winning.
About Nazimuddin, he picked Nazimuddin ahead of SN. It's his choice, if Nazim fails in a position where he never played, Siddons should share the blame.
Batting is a team effort. One or two players are probably doing well, but we are not getting contributions from tails or other batsmen as he already managed to kill the fire. And that's why the overall batting picture looks gloomy.
Another important thing is the way runs are coming. They are coming for losing causes. In Dav's era they were coming for both winning and losing causes. Now it's your choice which cause you rate more!!