Sub menu drop down hidden at bottom of menu – css z index?

Hi there, when I open a menu item with a sub menu dropdown that’s close to the bottom of the tob bar in which I display the menu, the sub menu dropdown links disappear behind my other content.

I’ve tried to use the z index in css to bring the drop down over everything else but I must be doing it wrong somehow, could anyone drop me the correct css code to achieve this?

Thanks in advance