Dropping issue

Hi,

I have an html page with 2 'div', with different size border, in it, let's call them D1 and D2. D1 is the father of D2 (so D2 is in D1) and they're both 'droppable'. The problem is when i want to drop a new div in D2, it always drop it in D1.
I think that i miss a droppable option.

Someone can help me?