The Hollywood Reporter has done a very quick post-mortem. It covers the casting of course, but what appear to be the real issues were the lack of clarity in the marketing and (for a change) the reviews.
People didn't know exactly what the film was going to be and so;
Paramount could tell by the Google searches it was monitoring that in the final week before the film opened, potential moviegoers were checking out reviews before making a decision in higher than usual numbers.
Initially, the film’s Rotten Tomatoes score rose as high as 71 percent positive reviews, based on the first enthusiastic fanboy reactions and critiques from Europe. But as the week progressed and more reviews poured in, the movie’s score began to plummet, eventually falling to 42 percent. Ghost’s tracking, which initially suggested the film might open in the high-$20 million range, maybe even cross over into low-$30 million territory, hit a wall.
“It was very Rotten Tomatoes-sensitive. Lots of people were seeking out information about the film,” says Colligan. “And we got hit pretty hard by the print journalists.”
So if you want to make a complex story, don't spend that much on it, or if you spend that much, simplify the concept.