Find and Find&Replace don’t always find every instance. I’ve had multiple occasions when I’m searching for something and I know the search term is in my document, but Find doesn’t pick it up (so I have to myself. Tedious). Sometimes closing and reopening the file improves matters. It’s not a format or match case, etc, problem, and to test things, I’ve opened documents in Google and Word and they seem to find things when Writer doesn’t. I’m using 6.0.7.3.
I occasionally experience the same, even on current versions of LO.
If you expect any suggestion to improve your case, edit your question to give more factual elements about your search target. Don’t use a comment, modify the question. Mention OS name and, most important, save format (.odt vs. .doc(x)).
Give an example of a failure (search key or expression). Describe as accurately as possible a missed occurrence. Are there non-printing characters here, like soft-hyphen, word joiner or other special formatting characters? Does it happen frequently on characters with diacritics (accents)?
… and maybe the searched text is contained e.g. in fields (tdf#70142)?
Not a solution but a notice that I also have this issue. I am writing a story and I used the word confessed several time in it, but the Find function can only find one instance of that word. If I search through my document manually I can find every time I wrote it. I wrote the document in LibreOffice so it’s not an import from Word.