Searching with the Search tab only looks in 3 of 19 merged chms, no matter what is searched for

It would be worth checking that the window and single source layout properties of all 19 projects are setup exactly the same. Also make sure that there are no child CHM files in the project directory before you compile. RoboHelp insists that they are copied in when you add links but they can cause issues like this when you compile. When you compile there should be no other CHMs in the project directory.