A lot of people around my age (19) grew up with the movies and the books, but those were being worked on close enough to each other that they could appeal to the same groups simultaneously, more or less. The Philosopher's Stone movie came out only 4 years after the book, and the same for the Chamber of Secrets, and the Prisoner of Azkaban. Neither medium needed to adjust all that much to any age difference in their audience. The series is still fresh for everyone involved and those who got interested early are still invested as there were still new books coming out while the movies were being filmed.
Comparing that to the Percy Jackson series, you have the first movie coming out a year after the book series had concluded (the first part of it anyway). Everyone who had been reading up to that point had both aged up with the story and seen the story concluded. The movie was old news to most of the people that would have been enthused about it a few years prior. I saw the first PJ movie on a redbox maybe a year or two after it came out mostly just out of curiosity. I had finished the books a long time before then and didn't really think the movies had anything to offer. Maybe if I was younger at the time or the books hadn't all been written I may have had a different reaction. Writ large, having the core audience for your work uninterested from the jump can't be good for success.
TLDR; If you want the series to do better, the movie deal is going to have to come earlier or the books later. The movies started after the books were done and very close at that. Finished stories that are still fresh in people's minds aren't good fuel for adaptions unless you can guarantee the films will bring in a wider audience. ASOIAF was able to expand very far from an HBO drama, but the same growth of popularity can't be counted on for YA fiction; especially when there's so much of it. To get big the movies have to cater to their core audience which at first will be mostly readers and their families, and the first step to that is making sure those people are still invested in the property.