HTML version won't browse index subentries

Our document will primarily be used in the html version so we really have to make sure users can search for their problem. We realized quickly that using the regular serach will usually not solve the problem (especially if the keywords used are broad or the user doesn't know the correct term) so we decided to optmize our index to display the correct results.

Now we are the ones experiencing problems: If we browse the index, it won't show sub entries. Ever. It seems like it only browses the first level.

Example: In this case, my search for "aktiver Dozent" will show no results because it's a second level subentry, only "Umfrage" (the first level) is browsable.

Is this a bug or is there anything we can do about it? Do you have any alternative solutions (such as working with search metadata)?

Edit: Used version: FM 15