A series has more substance if each volume has an independent plot, even if it’s part of a larger conflict. A reader who picks up a book out of order should be able to follow the action while still wondering what happened earlier, which is likely to drive them to read earlier stories eventually.
It’s a good idea to increase the plot’s complexity as the story progresses. Without further obstacles and challenges, your story can get boring and predictable. I loved reading “The Hobbit” series, but when I saw the first movie, while I admired the visuals, special effects, and casting, I got bored after about an hour thinking, “How long can I stand to sit here watching monsters chasing hobbits?”
I’ve also read stories that just went on and on and on with too many obstacles. They can have a “ho-hum” factor as well.