What you need to do is read more books, especially series books by writers who know how to do this kind of writing.
Analyse each book, break down the scenes and what happens in each that leads to the next plot point. I am guessing the target audience for this is mid-grade kids, and that's the part of the library you hang out in.
Watching a movie might help a little, but you are working on a book, not a film. Two different critters.
You do not need to get complicated about no one understanding the clues for years on end. Have the protag
discover a long hidden clue in the first place. Turn it into a race from plot point to plot point, not a puzzle-solving campaign. In
Treasure Island, more than one set of characters were chasing after the MacGuffin.
Base the story on
characters, not the plot. People fall in love with characters, not a plot.
Read at least 100 books for each one you plan to write. If you go, "Hot puppies, read only 100 books? I can do that easy-peasy!" then you have the fire in the belly to do this. If it sounds like a horrible chore, then take up paint by numbers or quilting instead. All great writers are insatiable readers. The library needs to be your new second home.
Start with these:
Terry Pratchett's kid's books:
https://geekdad.com/2015/03/cool-books-terry-pratchett/
Johnny and the Bomb is especially good as modern kids figure out how to save one's grandfather while he's a child as they time travel back to WW2. All that and he got the physics right!
https://www.amazon.com/dp/B00ADQDFLW/?tag=absowrit-20
Steven K. Smith's mysteries:
And anything Nancy Drew. Don't turn off on these. I am reading them 50 years later and they hold up.
Sherlock Holmes. Yes, look at those classics. Learn from a good mentor.
The Genius Files by Dan Gutman -- five books, all entertaining
[URL]https://www.amazon.com/dp/0061827665/?tag=absowrit-20
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