Search yields no results

When I converted an HLP project from a previous version of Robohelp into a CHM project in Robohelp 6, the Search function worked properly, displaying the titles of all topics in which the keyword appeared. When I used some of the same RTF files to create an alternate project, the Search only returns a "No topic found" message. As far as I can see, all of the project and compile settings are the same. What am I missing?