I have following simple code:
<%@ Page Language=\"C#\" AutoEventWireup=\"true\" CodeBehind=\"testForm.aspx.cs\" Inherits=\"Orbs.testForm\" %>
I had the same problem, but I solved it by calling onindexchanged function manually, like this:
onindexchanged
ddl_SelectedIndexChanged(null, null);
I know this may not be the perfect way but it's working for me.