The search box is autocomplete-enabled. If you don't use that function (e.g. you continue to type your word or phrase without selecting a term from the drop down list) your search may not work if it has a spelling error or extra space.
If you have selected a chapter from the Table of Contents then your search will only be conducted within that chapter. If you would like to conduct a full text search make sure that none of the chapters/sections are selected.