I use Finder's Search bar a lot to find specific items nested in my folders. It usually works flawlessly and I find all the items I want. Other times it just doesn't do anything:
Imagine a folder with just 10 files, and one of the is an image ending with
.png. If you search
.png in the search bar, Finder literally doesn't find it!
This happens to me maybe once every couple days. I'm not sure why - relaunching Finder doesn't seem to help either.