Disable next button until timeline ends and layers revealed

Hi 

I have a slide with audio on the timeline and two images with triggers to reveal two layers. The client wants the course locked down until all elements are viewed.

The course is set to Restricted.  

To lock the screen I have:

A screen complete variable (screencomplete) set to false. 

A trigger - change state of the next button to Disabled when timeline starts 

A trigger - change state of the next button to Normal when screencomplete (the variable) changes

A trigger - Adjust variable screencomplete to true when timeline ends on condition image 1 state is Selected and image 2 state is Selected.

This works fine going forward in the course. The button is disabled until both the timeline completes and both images are selected.

However, if I return to the screen using the Previous button, the Next button is disabled. I have tried building with 'When revisiting' set to 'reset to initial state' and 'resume saved state' but the button is disabled in both setups.