z-index with IE 7

Question

I have a main div ("myBubble",z-index: 1001) wraps 2 span(z-index: 1000,z-index: 1002) and a svg (z-index: 1000), and I have an other div ("modal",z-index: 999) position behind the main div by set its z-index. The problem is: in IE8 I can make main div position at the front of the modal div, but not in IE7. I can make it work on other browsers and IE8, but not IE 7. Could any one have a solution for this?