search in folder returns nothing

Sometime recently searching in folders stopped working for me. Using the CMD-F or Search box in a finder window to do a search on This Mac will return results, but when I click to change the scope of the search to the current folder, I then get "0 items", even if it is a file I can see before I start typing.

I have checked Finder Preferences and there is nothing in the Privacy setting.

I remember long ago hearing about "re-indexing spotlight" and maybe that is what I need to do? But how? I would try it.