asp.net 点击按钮，页面没有任何变化，后台代码不触发

------案例如下，没有细细研究 ValidationGroup怎么用
<asp:DropDownList ID="ddl" runat="server" Width="154px"
                        AutoPostBack="True" OnSelectedIndexChanged="ddl_SelectedIndexChanged"></asp:DropDownList>

<asp:CompareValidator ID="cvabc" runat="server" ControlToValidate="ddl" ValidationGroup="save"
                    ErrorMessage="<%$ Resources:xxx, xxx%>" Operator="NotEqual" ValueToCompare="-1">*</asp:CompareValidator>
<asp:Button ID="btnSave" runat="server" class="button" Text="<%$ Resources:xxx, xxx%>" ValidationGroup="save" OnClick="btnSave_Click" />