Ah, I thought that title would get you to scratch your head, wondering what the heck it means... so let me tell you, okay?
I coined the term "Ghostbusters Syndrome" to define when a successful movie comes out, spawns an animated series spin-off, and then, after that animated series has been out for a while, they finally make a sequel to the original movie that, well, pretty much blows chunks, and doesn't do anywhere near as well as the original did.
So far as I know, this syndrome is always true -- although I can only think of two instances off the top of my head (of animated series that spun off of movies, that is). Well, maybe three.
First of all, of course, there's Ghostbusters, or rather, The Real Ghostbusters, the animated series that originally aired on ABC-TV Saturday mornings, and also ran five days a week in syndication. This animated series was very good, expanded upon the original movie premises, did more with the characterization, and did an awful lot of episodes... taking the concept into areas that I'm sure that Bill Murray & Co. never ever considered (okay, maybe Murray didn't originate Ghostbusters... maybe it was Harold Ramis, or someone else entirely. Just go with me on this, okay?).
Finally, they made Ghostbusters II -- I think that the animated series had been on for several years by this point (hmmm... let's see, the first movie came out in 1984, the animated series came out in 1986 and ran until 1991, and Ghostbusters II came out in 1989), and there were a lot of stories told (and obviously, there were two more years of stories to tell, too)... and I think that there was no way by that point that Ghostbusters II could've met anyone's expecations whatsoever.
The animated series just raised the bar way too high.
The next most prominent instance of "Ghostbusters Syndrome" is "Men In Black."
The movie version came out in 1997, with the animated series following hard on its heels the same year. The sequel came out in 2002 (maybe the thing to bear in mind, too, is that you shouldn't wait five years to make a sequel). Where it differs is that the animated series apparently ran its course in 2001 (makes me wonder if the movie people got the show cancelled so it wouldn't compete with the movie sequel?).
Again... the animated series expanded on the movie concepts, added more to the characterizations... and it was also a lot of fun, and brilliant in many respects.
Of course, the sequel to Men in Black... well, it sucked.
The only other instance of this that I'm aware of would be The Mask... but that could be pushing it, because I don't think that The Mask animated series lasted that long at all, and of course, the Son of the Mask was a sequel pretty much in name only, lacking any of the original cast at all (so far as I'm aware).
Now, there are exceptions... there's always exceptions. I'm sure that someone reading this will mention that after the first Spider-Man movie, as well as after the first Batman movie (the Michael Keaton one), that there were animated series, and yet both franchises had successful sequels...
But honestly -- neither animated series was directly based on the movies, but were based more on the comic books, with some nods to the movies. (You could even make the same argument for X-Men: Evolution as an animated spin-off of the X-Men movies, but I wouldn't).
Now, isn't it funny that the only exceptions seem to be movies based on long-running comic book series? Yes, okay, Men in Black began as a comic book (although one could argue that the movie was only vaguely based on the comics, which not only dealt with aliens, but also supernatural stuff, i.e. monsters and their ilk).
If there are any other examples of this any of you can come up with, feel free to mention them -- even if there's a possible *real* exception to the rule!
(Oh, I just remembered... after Robocop there was a Robocop animated series that also didn't last long, and of course, we know the Robocop sequels weren't that great... I don't know if Swamp Thing II came out after the Swamp Thing animated series, either).