Gimmicks.
Special effects and gimmicks only work - and can work superbly - if they are in support of an already good story; once they are the story, and not the supporting element - the thing loses its raison d'être.
The original "Shrek" movie worked because it was clever, funny, well told, with a strong story and compelling characters given great dialogue in a stunning script. The fact that the artwork and CGI were superb added immeasurably to one's enjoyment of the movie, but - by themselves - they wouldn't have been sufficient to lift it (as the sequels subsequently demonstrated) had the story, characters and script been found wanting.
This is what I have been saying for quite some time, and what do you know, the box office numbers are saying the same thing. If that is ALL you have, well don't be surprised if people do not want to see it.
For some strange reason you can usually tell just by the trailers!