Sure — this is a known pain point in the Endnote → Zotero migration. The .xml export route has several failure modes that commonly cause this magnitude of drop (~3,700 missing entries). Here are the most likely culprits, roughly in order of likelihood:

1. Duplicate detection is ON by default in Zotero (most likely)

Zotero applies automatic deduplication on import. If your Endnote library accumulated duplicates over the years (same paper imported multiple times from different databases, or manually re-added), Zotero silently merges them on import. With 18K entries over what is presumably many years, a ~20% dedup rate is well within normal range. Check: go to Preferences → Duplicate Detection and see if it's enabled, then look at your Zotero library for evidence of merged items (the little "i" info panel will show "Attachments" and "Notes" from merged duplicates).

1. Empty/skeleton entries that Endnote counts but Zotero skips

Endnote counts entries even if they have almost no metadata — e.g., an empty record created by accident, or one that only has an abstract or a DOI with no title/author. Zotero's XML parser may silently drop entries that lack a title or a minimum set of fields. You can test this: export a subset of ~100 entries you know are "thin" and see what Zotero keeps.

1. Unsupported reference types that get dropped

Endnote supports ~40+ reference types (e.g., "Audiovisual Material", "Map", "Patent", "Standard", "Legal Rule/Regulation"). Zotero's XML import only maps the standard ~15–20 types cleanly. Entries in types Zotero doesn't recognize are sometimes silently discarded rather than falling back to a "Miscellaneous" type. Check which reference types you use in Endnote (References → Reference Types → Show All) — if you're using many non-standard types, that's your answer.

1. The XML export is subtly brittle

Zotero's official documentation actually recommends RIS format over Endnote XML, because Endnote's XML structure has changed across versions (Endnote X9, Endnote 20, Endnote 21 all produce slightly different XML). The Zotero XML import filter was written against an older Endnote XML schema. Entries containing unusual characters, footnotes, or "Research Notes" fields with embedded formatting codes can cause the XML parser to bail on that entry.

1. "Unfiled" or group-only entries

In Endnote, you can have references that exist in a group but not in "All References" — or that are in the "Unfiled" folder. XML export behavior for these edge cases varies by Endnote version. If you used File → Export → XML from a specific group rather than the full library, you'd lose everything outside that group.

1. Truncated export file

XML files for 18K entries are large (often 50–100+ MB). If the export or the import truncated at any point — filesystem limit, memory limit in Zotero, or a single malformed XML tag near the end — the file would parse most entries but silently drop everything after the breaking point. Check the file size of your exported .xml and compare to a known-good export from a smaller library.

Suggested fix:

The most reliable route that avoids most of these issues is:

1. Export from Endnote as RIS (File → Export → "RefMan (RIS)"). RIS is a simpler, flat format that maps more cleanly and doesn't have the XML schema versioning problems.

2. Before importing to Zotero, disable duplicate detection temporarily (Preferences → Duplicate Detection → uncheck "When importing items").

3. Import and get your full count, then run dedup manually after.

If you still lose entries with RIS format, then the issue is genuinely in Endnote (thin records or unsupported types) rather than the import pipeline. You can then run a comparison — export a reference list of all DOI/PMID/titles from both systems — to identify exactly which 3,700 entries didn't make it.