My USA English Thesaurus works in most of my documents, but ...

… it doesn’t work in one particular document. What’s causing that problem, and how can I fix it?

If the problem is about a particular document, share it.

Is the document in a different language, e.g. English (NZ)? Or is it set to [None]? In the menu click Tools > Language > For All Text > English (USA) spell check and Thesaurus should be operational again in the document.