The problem is in the screenplay, and a screenplay is a movie's spine. Without a healthy spine, next to nothing else can work properly. Barring a few early gags that illustrate the 'busters fall from grace (one hosts a tacky talk show; others have resorted to goofing around at kiddie parties), the guys are given nothing to do. The characters don't evolve adequately or interact convincingly. The banter feels forced and the smiles look like plaster. One of the best assets of the original GHOSTBUSTERS was its loosey-goosey approach; the sequel feels like a blueprint concocted by the bean-counters at Columbia Pictures.
A few stray nuggets of sustenance can be found: Peter MacNicol does a very good job as a loony curator, even if all he's doing is acting nervous while showing off a really goofy accent. Rick Moranis actually outearns Murray and Aykroyd in the laughs department, and he does so in about 1/4th of the screen time. Unfortunately, the performances by Murray, Ramis, and Sigourney Weaver feel like bad dinner theater. The actors' eyes have the look of people punching a time-clock, rather drably uninvested in the project they're working on.
Every 4 or 5 years I give an old disappointment a second (or third) chance, but I'm officially retiring GHOSTBUSTERS 2 from that rotation. True that it's not as awful as something like BLUES BROTHERS 2000 or a MEATBALLS sequel, but GB2 feels like a hollow and (ironically) soulless sort of cash-grab -- a "can't miss" sequel that was rushed into production with a weak plot, a lot of wheezing banter, and actors contractually required to earn huge pots of cash for a project they probably didn't even like.