As someone who read the books to death as a kid, and as someone who was a good little Catholic boy then and is an atheist now, I was expecting to have a kind of mixed response to the movies.
As it turns out, I quite liked The Lion, the Witch, and the Wardrobe, and I didn't care nearly so much for Prince Caspian. Just before each film premiered, I reread the book in question (for the first time in quite a few years). And what I hadn't remembered is that Prince Caspian, the novel, is an almost plotless mess. The most interesting part of PC as book and film is the beginning, as the Pevensies explore a changed Narnia and discover what's happened. The book has some deeply silly sequences, such as Aslan and the girls leading a wild pagan romp, complete with the Roman god Bacchus. The movie, which is apparently intended to be Christian entertainment, dropped all the pagan elements of the book and added a surprisingly violent battle sequence (the assault on the castle) that appears nowhere in the book and feels very out of place in a sequel to the relatively kid-friendly Lion.
So what you've got is a dark and often depressing movie with relatively little fun or wonder and a lot of LOTR-wannabe battles, a film that seems very different from its predecessor in tone, violence, characterization, and style, and seems to want to be something it isn't. It's not the same kind of mess PC the book is, but it is nonetheless a mess, and it lacks the charm the first film had. It's not really surprising it didn't do as well. Now, as I remember it, Voyage of the Dawn Treader was full of fun, wonder, and excitement, and a well-made and faithful movie could do pretty well. I hope it's made some day, but not necessarily written by the same people who adapted Prince Caspian.
(Incidentally... for all the fuss about the Chronicles of Narnia as Christian allegory, they're rather more complex than that; there's a lot of pagan and classical mythological influences in there. It's possible to read many of the books and not even pick up on the allegory, especially when you're young; I'm not sure I fully got it until The Last Battle, and even as a good little believer I didn't like the shift from covert allegorizing to overt sermonizing in that book.)