I think as always there's a lot of book to try to squeeze into a screenplay.
The best screenplay writers cut novels down ruthlessly and knit the rest together well - the worst cut or change the wrong bits, sometimes try to squeeze too much in and are poor knitters - leaving the foreshadowing in after cutting the payoff.
Bet it would have been better had Adams been around still, seeing as he'd already successfully got it into several different mediums!