I think that is because Flip usually uses them wrong. The best way for the subs to get going is to leave your starting PG with 3 -4 subs. That way you got a game manager that will control the tempo and feed those guys to easy baskets and they gain confidence thrpugh that. Flip usually brings Stacky as a second - third sub and rarely, unless it garbage time, uses 4 subs together.
That is way it seems that when the starters are good, the bench is good. It is because than it is garbage time and Flip can give his starters rest.
Now look at Boston or Lakers. I have seen Celtics in a tight game going thrugh 4 munites in second quarter with Pierce and four subs many times. That allows you to see, who from the bench is hot and gives them confidence.
Lakers the same. Kobe + 4 subs isn't rare sight at all in Staples Center and even more so, this is the time that Kobe does Jordan's thing and just rests on the floor and barely touches the ball.
I dislike Flip as a coach and wouldn't give him a job in NBDL, not to mention NBA and that is also one of the reasons.
By the way, does anyone remember him using a bench in Minni? Does anyone remember him building any player over the years (Garnett doesn't count)?