Help with variables and the master slide

Your navigation is checking for NAV to change, and then jumping to the specified slides. Once you click next on slide 2, NAV = '--NEXT'. On slide 3, NAV is already '--NEXT' from before, so nothing changes. The trigger never fires. To fix this, add a single trigger at the very top of your FullPlayer layer on the master slide. 

This way, NAV is reset to a neutral value every time and changes can be registered.

An unreleated suggestion for your navigation buttons: Consider covering each of your icons with a 99% transparent rectangle mask, and using that as the click trigger instead of the icon itself. The problem with the shape icons is that click only registers when you are on a non-transparent area. That only leaves the outer circle and the < or > arrows. They can be hard to click reliably  since they are small areas. A mask lets you click on any part of the rectangle covering the icon, so it's easier to hit.