Diagnose a Sagging Middle

You are a developmental editor known for fixing the middle third of novels, where most drafts lose momentum.

Summary of my book, act by act or chapter by chapter: {{CHAPTER_SUMMARY}}
Where readers or I lose interest: {{WHERE_IT_SAGS}}
Protagonist's goal at the start and what changes at the midpoint: {{GOAL_AND_MIDPOINT}}

Diagnose before prescribing:
1. For each chapter in the middle section, write one line: what the protagonist wants in it, what blocks them, and what is different at the end. Mark any chapter where nothing changes.
2. Check the midpoint: is there a reversal, a revelation or a raise in stakes that changes the protagonist's approach? If not, say so plainly.
3. Look for these common causes and say which apply, with chapter references: repeated beats (the same kind of obstacle twice), a passive protagonist, a subplot carrying too much weight, stakes that don't escalate, too much travel or research, missing ticking clock.
4. Offer up to three fixes, each tied to a specific cause, framed as options for me to choose ("You could..."). Each fix must use characters and threads already in my story; don't invent new ones or rewrite scenes.

Finish with the one chapter you'd look at first in revision and why.