Activities window search not finding files

The Activities search seems to be erratic. Sometimes it finds files, sometimes it doesn't. It would find files that I used recently, but otherwise, it finds some obscure files but not the obvious ones I look for (such as text documents). Unlike the Activities search, a search from a folder app brings desired results.

I know the problem is hardly defined but I don't have any other clues of why it happens and how to verify this behavior.