Summary
Layout > Table of Contents fails to generate entries for a specific chapter document ("1Part 1FTCIT 3RD ED.indd") within a book project, even though the target paragraph style is confirmed correctly applied, the TOC style's Include list is confirmed correct, and the issue persists after rebuilding the TOC style from scratch, rebuilding the document via IDML export/re-import, and testing with brand-new, untethered content. A control test in a brand-new, unrelated document with a brand-new paragraph style and TOC style generated correctly on the first attempt, ruling out the application installation/preferences as the cause.

Environment
Application: Adobe InDesign 2026
Build: 21.4.0.74
OS: macOS Tahoe 26.5.2
Project type: InDesign Book (.indb) with 9 linked chapter documents, synchronized paragraph/character styles

Expected Behavior
Running Layout > Table of Contents with a paragraph style included in the TOC style's "Include Paragraph Styles" list should generate a TOC entry for every paragraph using that style — whether scanning a single document or, with "Include Book Documents" enabled, across every chapter in the book.

Actual Behavior
When generating from the book's Front Matter/Preface document with "Include Book Documents" enabled, only two of nine chapters (Part 7 and Back Matter) produce entries. The other seven, including Part 1, are silently omitted — no error, no warning, just missing entries. This was traced down to Part 1 failing to generate any TOC entries even in complete isolation from the rest of the book.
Isolation Steps Taken (in order) and Results
Confirmed paragraph styles are synchronized and named identically across all chapter documents via the Book panel's synced-style indicator and manual inspection. (No issue found.)
Saved all chapters via "Save Book" to rule out stale/unsaved content being read by TOC generation. (No change.)
Checked Book panel status icons for broken links or "needs saving" flags on all chapters. (All normal.)
Searched for duplicate/similarly-named paragraph styles (e.g., a style nested in a group vs. a flat style of the same visible name) using Find/Change with Format-based search, scope "All Documents." Confirmed the target style ("Chapter Titles") is found correctly in every chapter, including Part 1. (No duplicate found.)
Checked layer visibility, conditional text, and master-page overrides on Part 1's chapter title frame. Layer visible, conditional text set to [Unconditional], frame is a local page item (not an un-overridden master item). (No issue found.)
Checked Book panel > Synchronize Options to confirm "TOC Style" was included in the sync categories, then re-ran Synchronize Book. (No change.)
Rebuilt the TOC style's Include Paragraph Styles list by removing and re-adding the target style fresh from the document's own current style catalog. (No change.)
Isolated Part 1 completely from the book: generated a TOC from within Part 1 alone, with "Include Book Documents" unchecked, so the scan could only read Part 1's own content. Result: empty — not even Part 1's own chapter title appeared, despite the paragraph being confirmed (via clicking into it with the Type tool) to carry the correct paragraph style, and despite Include Book Documents being off (so no other document could be responsible).
Tested a brand-new, untethered text frame inside the same document, with freshly typed text and a freshly applied instance of the same paragraph style. (Still empty — rules out frame/thread-level corruption of the original title frame, a known InDesign issue with a similar symptom.)
Rebuilt the entire chapter document via File > Export > IDML, then reopened and re-saved as .indd. Re-ran the isolated test. (Still empty — rules out low-level document corruption that an IDML round-trip would normally clear.)
Built an entirely new, independent TOC style from scratch ("New," not "Edit"), including only the target paragraph style. (Still empty — rules out corruption in the original TOC style object itself.)
Confirmed via Select All (Cmd+A) inside the empty TOC result frame that no invisible/mis-styled text was being generated — the frame is genuinely empty, not a display/formatting issue. (Confirmed empty.)
Control test: created a brand-new, completely unrelated document with a new paragraph style and new TOC style, no connection to the book project. Result: generated correctly on the first attempt. (This rules out the InDesign installation and preferences as the cause — the bug is confined to this specific project/document.)
Notes
Because a from-scratch document reproduces correctly, this does not appear to be a universal, easily-reproducible bug — it appears to be tied to some state specific to this document or book file that we were unable to identify or clear through normal means (resaving, resyncing, style rebuilding, or IDML export/re-import).

Several users have reported the same core symptom (paragraph style confirmed correct, TOC style confirmed correct, entries still silently missing, sometimes traced to text frame threading) in Adobe's own community forums, without a definitive root-cause fix:
community.adobe.com — "InDesign generates an empty table of contents" (thread 868203)
community.adobe.com — "Table of Contents missing paragraph styles" (thread 11722707)
community.adobe.com — "Adobe InDesign TOC Not Populating" (thread 13721262)
Source files (the .indb and affected chapter .indd files) are available and can be provided to Adobe engineering to aid reproduction, if requested.

Impact
Blocks generation of a working, complete Table of Contents for a 9-chapter book project ready for publication.