the middle book can drag bc you’re past the inciting event and initial worldbuilding that was designed to suck readers in and not yet at the finale. if it’s not planned really well, readers can feel like they’re just watching things get built up or dragged out with no payoff until the next book
Muddled middle is a big problem for writers who don’t know what they’re doing. It happens in individual books and can be worse with series. But especially if the author knew ahead of time they’d be writing a sequel, that helps. Sometimes they’re scrambling for plot points one half of the second book and trying to set up events for book 3 the other half, and it shows.