Sub-menu of Navigation Bar Hiden by slideshow

Hello,

The sub-menus in my headers navigation menu are hidden by my slideshow. The slideshow is located below the header navigation menu. I am using the twenty twelve theme. I changed the z-index from 1 to 2 in my child theme style.css file. My sub-menu now appears over the slideshow, BUT when the slideshow changes between pictures the sub-menu is hidden again.

Does anyone know how to make the sub-menu appear in front of the slide show all the time, even when the picture are changing?

Thanks!

Before
.main-navigation li ul {
display: none;
margin: 0;
padding: 0;
position: absolute;
top: 100%;
z-index: 1;
}

After
.main-navigation li ul {
display: none;
margin: 0;
padding: 0;
position: absolute;
top: 100%;
z-index: 2;
}