I have two Help file Projects and both are to generate as .CHM with Context-sensitive mapping. For one of the .CHM, the context-sensitive works. For the other, it does not. Both .CHM files are identical with slight changes in terms of the contents and they can be used for the same program because the Map IDs are the same. It is just wording in the topics that differs.
I have checked on the .hhp file and I see that the ALIAS and MAP sections are there and the Map ID files (with extension .h) are also there.