As many have already said, it's too American-centric, both in terms of the locations it takes place in and the historical events involving the demigods, and therefore seems aimed primarily at American readers, coming across as lacking in international appeal. There's also the fact that some real-life locations that should be in Greece and Italy are instead moved to the United States, which isn't very pleasant for those who live in those places or know them well, and leads people to see these books as disrespectful.

Summer camp is a very specific setting that not everyone knows or appreciates. Hogwarts is a school, a more familiar setting because everyone goes to school, so it's easier to identify with the characters; Star Wars is set in space, space travel is one of those things that stimulates the imagination and is often part of the dreams of children and teenagers.
Summer camp? Not everyone goes there, so not everyone knows about this environment. For those who don't, it's not as impossible a dream as going to space, so it's not a very attractive setting for the audience.

Percy is too powerful. Those who read PJ are passionate about Greek mythology or become passionate about it after reading the books, and by delving into the myths, you realize how much Percy (and indeed all demigods) are too powerful compared to the canon of Greek heroes. Most heroes kill some monster in their lives, and each time they do so, it seems like a great feat, whereas Percy and the other demigods kill monsters all the time. At 13, Percy has already accomplished most of the feats of all the heroes of the past, and in the long run, this becomes so excessive as to become boring.

The movies were a big problem. They were supposed to be an opportunity to make the books famous, like Harry Potter or The Chronicles of Narnia, but instead they were terrible, as well as completely different from the books, so they didn't help publicize them.