Formulas for variables

Hi guys,

There is an element of the variable triggers that is really confusing me (heads-up that I'm new to triggers).

I set up a boolean variable, and it was successfully changing from 'false' to 'true' when the user got the correct answer in a MCQ slide.

I then wanted the relevant checkbox to change to a 'ticked' state, based on the boolean being 'true', on my Scenario Checklist slide. 

On this slide, I set up the trigger:

Variable Trigger: Change state of [Checkbox 1] to [done] when [S1 correct] changes.

This trigger did not successfully change the checkbox to 'ticked' and I can't understand why?

However, after a long while troubleshooting, I realised that this trigger works:

Slide trigger: Change state of [Checkbox 1] to [done] when the [timeline starts] on [this slide] If [S1_correct] = value [true].

Why does the first trigger fail and the second one work?

I feel like I should be able to set up a simple trigger like:

Change state of [Checkbox 1] to [Done] when [S1_correct] changes to value [true]

but this option is not possible; it's not there in the list of available options. Why is that?

I've attached my slides here if that helps. 

Thanks!

Jess