~ Happy reading ~
Yes, fantasy books and movies didn't fit most of the time. There are a few exceptions from that rule but a movie seems to be simply to short for showing the worldbuilding, the story and the character development of a fantasy book. Production companies should go with tv format - this could be truly successful (e.g. Games of Thrones or True Blood)
The Lightning Thief by Rick Riordan
I love Percy Jackson and his friends. I adore the mixture of greek mythology (in the new series also roman mythology) and fantasy, of normal teenage life and weird half-god problems, of action scenes and funny elements. The film? Well, it's crap. Most of the unique humour went lost because of more action. The characters became stereotypes. They looked wrong (especially Annabeth and Percy) and of course a lot of the story was wrong or missing. It must have been a success otherwise a second film didn't make any sense. I have safed myself the bother^^.
Eragon by Christopher Paolini
This isn't a favourite of mine although I enjoyed the first book of the Inheritance series. But have you seen the film? Urghhh. It was horrible. The only bright spots were the bad guys, e.g. John Malkovich as Galbatorix, Robert Carlyle as Durza and - my favourite - Garrett Hedlund as Murtagh. Worst was the translation. Maybe you know the song 99 red ballons from the early 1980ies? The singer, Nena, spoke the German voice of Saphira which was unbearable. If Nena doesn't sing her voice is high, prickly and childish. Her voice isn't in the trailers otherwise I would have given you a sample - but maybe it's better to leave your ears like this^^.