Why can't Finder find some obvious items sometimes?
Indeed, the Finder fails to live up to its name in my experience as well. Results seem so frustratingly random as to what is found and what is missed. 

Beware of one setting that may help: Finder > Preferences > Advanced > "When performing a search` with a popup menu offering "Search this Mac", "Search the Current Folder", "Use the Previous Search Scope".



Beware of the "Search" bar with scope filter buttons that appears when typing search criteria into the upper-right corner.



Finding the Finder nearly useless for finding, so in the past I have in the past purchased a license to Path Finder, a Finder replacement/supplement app. There are other such apps as well.

Another option is using command-line tools in Terminal.app. For example, this article shows the use of the find command. See also this Question on SuperUser Stack Exchange, How do I find a file by filename in Mac OSX terminal?.