Three Problems With Middle Novels

The Unfinished Song series is half-way through. Not coincidentally, I've been obsessing lately about how to write solid "middles."
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:
1. Filler
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 lit...