Searching image files for text with Finder

When I photographed some book pages not so long ago, I was thrilled to find that Finder very effectively found text on the pages.

Since then I've run into a problem and would love some help figuring out what I need to do differently. It soon became apparent that the search will only work once. Once viewed through Preview the document drops out of the Finder search results before your eyes and won't appear in any future results.

The only way I can get the file to re-appear in a search is to add the desired word into the Comments box of the file's Get Info page. What can I do differently to get the feature to work repeatedly?

This is such an amazing feature if I could work out how to use it. Any help much appreciated.