In the Search dialog box, check the bottom of the dialog box for filename or other criteria, possibly left from a previous search, that could be excluding the file you are searching for. Check that the file was indexed. Check that the file contains the word you are searching for.
Some files contain images of words rather than the words themselves.
For example, a PDF file may contain an image of a scanned page but no text. Similarly, an HTML file may display text in a banner image. To convert images into searchable text, use an OCR product such as Xerox TextBridge, Adobe Capture, or Visioneer Pro OCR 100. Copyright 1995-2001 dtSearch Corp.