Why does the Windows search engine refuse to search inside specific .docx files?

Question

Windows

It is necessary to find among a large number of .docx documents those that contain the word I need. The standard Windows 7 search engine can do this and does it quite well.
But there are some documents in which I know for sure that this word exists, and they are not in the search results.
It is repaired by creating a new empty document and copying all the contents from that document there.
But I have a lot of such documents and I don't want to recreate everything.
What should I do?

Answer 1

Why does the Windows search engine refuse to search inside specific .docx files?

The folder containing this document is not included in the search.