You need to change your body to content sections. The master page works with contentplaceholders: your content pages need to provide the content for these placeholders see LINK
if your master page contain this code
<asp:contentplaceholder id="Main" runat="server" />
your page that Inherits from master should have
<asp:Content ID="Content1" ContentPlaceHolderID="Main" Runat="Server">
</asp:Content>