iOS Files app's search finds no files or only a few files

Trying to search by a file's name, or word in the file's name, but the search result never contains all the files, and often finds no files at all, despite at least some being visible in the list of files in the current folder. I have several locations: iCloud Drive, On My iPhone, OneDrive (several accounts, both personal and business). I can search for a file name that exists in all locations and still get "No Results." What is wrong?