Storyline 360 - Visited states seem a bit buggy

This is actually doing my head in a little bit.  I've got 2 x major projects I'm working on, and i can't get the consistency I need.

When I try and recreate the problem in a blank Storyline file (2 or 360), it works fine (see attached).  Then, over the course of creating the content, something happens, and it doesn't work any more.   I'd love to know what's corrupting it so I can avoid/fix it.  

I've stripped back one of my projects, to just the slide that contains the trigger, and it started to work again!  I'll keep trying to nail this and let you know how it goes.

My next step is to remove one slide at a time, and then layer by layer, and then object by object.  Very time consuming, but hopefully I'll come up with something.

Any suggestions are most welcome.