Haven't you noticed how sometimes, especially in a long series, some of the middle novels end up falling flat? Here are the three biggest ways I've seen series fall on their face in later novels:
When the middle novel/s seem like mostly "filler," the problem is that the characters are basically treading water in terms of plot. Sometimes, they characters literally spend whole chapters stuck in some place in the world, uncertain what to do... it's the author who actually has no idea what to do, but the characters are made to suffer for it. Sure, there are times characters mope for years, or centuries, depending on their lifespan, whinging they don't know what to do, but we don't really need to see this. I love the Twilight series handled Bella's three months of moping. Each chapter had the name of a month as a heading...and nothing else. Three months in a row, three pages. It conveyed her devastation and detachment perfectly, without making us suffer through it too.
Another failing of poorly-thought out middle books is that they become sloppy retellings of the earlier books. The characters go through the same motions again against a new villain, or new characters replay the same basic storyline as earlier characters.
Sometimes, an author uses repetition advisedly. Maybe a character is facing the same kind of problem because she didn't really grow as completely as she needed to when she faced it the last time, or maybe another character is having the same problem because that person needs to have a common cause with the hero. But this kind of deliberate echo usually resonates in a way that unthinking repetition does not. Most importantly, it advances the story in a way that mere repetition does not.
3. Jumping the Shark
Sometimes, writers trying too hard to avoid the first two problems veer off in such a different direction that what you love about the story is destroyed in the process. I actually find this worse than the first two. I'd rather race through a filler novel, where the heroine slays Son of First Book's Demon than have half the main characters killed off. (Unless you have already established from the start that Major Likable Characters Will Die, Suckers! *cough* G.R.R. Martin *cough*). The most important thing is to be true to the story: true to the characters, true to the world, true to the theme. Maybe I'm old fashioned but I believe an author should leave the dance with the Main Character she brought to the party.
My goal is to make every book in the series shine. Each one is a critical piece of Dindi and Kavio's story, none is filler. So I will be outlining the next six books exhaustively before I even begin the revisions on my trunk draft of Mask (Book 7). I now this will frustrate some readers in the short run, and maybe I'll even lose the impatient ones, but in the long run, the series will be stronger, better, and longer-lasting for it.