I totally agree with you, she's a great actress. And just because we like the books so much, doesn't mean they have to love it too. They do well on their jobs and make us happy and that's about it... Could you imagine if, to be on a HP film, you had to be a fan and know the books upside down? It would be tricky to get all the pieces you need to make a film together.
The girl who plays Luna (whose name I forgot, shame on me!) is but an excepction.