ShowDialog not behaving like a modal dialog in IE 7

Question

Hi,
 I have a usercontrol (say 'X') and a dialog (say 'Y') both having the same control (say 'A'. )Now this 'A' is a dropdownlist with some options. When clicked on one of the options, it calls SomeForm.ShowDialog(). This opens up a dialog which behaves like a modal dialog when called from 'Y' but the same dialog is not acting as a modal dialog when called from 'X'. This happens only in IE 7. I strongly feel it has to do something with the setting of window's handle. But I am not sure of how to resolve this problem. Please help.
Thanks,
Aisha