Rest of Project not working on re-launch

Hello,

I have created a custom timeline in my project by inserting several objects on a slide that are initially hidden and set to Display For Rest of Project.

Scripts are used at various points in the course to show the hidden objects.

Initial timeline looks like this:

Later in the course, it looks like this and the learner can use the buttons to jump to the various parts that have been completed:

Everything works perfectly if the learner does the course all in one sitting but if he/she closes the course part way through and then re-launches (in our LMS), the timeline does not function properly - the images and buttons that should be showing are not and the hint captions display for all the buttons (even though the buttons are not showing/cannot be selected).

Am I doing something wrong?  Should I be using a different technique for this? Shared Variables?