Check Boxes and Variables

Hello.  I am having some trouble in Storyline 2 (and I am terrible at variables).

I have some check boxes on a slide, contained within a number set, so only one can be selected at any time. 

If you check a box, it changes the relevant variable to 'true'. However, if you change your mind and check another box instead (ie automatically unchecking the original selection because of the number set), the variable seems to stay on 'true' even though it is not selected any more.

I have tried changing the trigger (click on check box, change the state of check box etc) but it still behaves the same. 

I have also tried changing the slide properties to 'Reset to Initial State', as the problem also exists if you revisit the slide and make different selections.  But no luck, it still remembers unchecked boxes that were once selected.

Has anyone experienced something similar, or can offer a better way to adjust a variable between true and false so it doesn't remember past actions?

Thanks, Chris