I can't at the moment look at your project, but this discussion has a sample that does what you describe.
The Restricted Navigation does not show the link to the next section until the previous section has been viewed. The Mixed Navigation allows the various sections to be visited in any order, but does not allow going to the End until all sections have been viewed. The Free Navigation allows the sections to be visited in any order, and the End to be visited at any time.
If you are currently using a trigger that shows the last button "when all the buttons are visited", that may be your problem. The "when buttons are visited" works well if you never leave the original slide (for example, if you are visiting layers, not other slides), but is not always reliable if you are going to other slides and returning.
