With the iPhone connected to the computer and the music tab selected on the iTunes/iPhone page, "Sync music" is checked and the radio button for "Selected playlists" is selected. In the window containing my playlists, several playlists are checked. If I click on "Apply," only some of the playlists are transferred to the iPhone.
Also, if I uncheck "Sync music," I get a warning that all music will be erased from the iPhone, but that doesn't happen either.
Sorry Ralph, not answering your Q but I have the opposite problem. For years I happily downloaded checked playlists on to my Nano. I'm now the proud owner of an iPhone and when I check playlists and synch many, but not all albums are downloaded to my iphone too. I cannot find how to stop this.