If there were an element with a higher z-index, why would making the lower container have a higher z-index than the default (such as 100) cause a change in action? That doesn't really make sense. Or am I misunderstanding what you are saying?
And what exactly do you mean by "active"? How are you testing for "active"?
In any case, make sure the browser debugger is open, right click on the location in question, and inspect the element. It should show you whatever element is at that location.
And,
read about z-index here.