The reason you can’t see the shadow is because the next div (#one) is directly below it, and the shadow is rendering beneath #one. Remove the background image from #one and the shadow becomes visible.
Add this to #intro’s CSS to make the shadow visible:
position: relative;
z-index: 10;
If you want shadows visible on the other text areas, you’ll need to adjust their z-index values as well, with the top element (#intro) having the highest value.