Required Field Validation not working in asp.net

Question

I have required field validation control for a radiobutton list. So if no values are selected then it gives me a error which is fine. But when i redo select something and click the button then it does't not fires the server event of the button. Once i have the validation erro then whatever i do it disable the server side event. any ideas why is it happening my code

<div id="studysub_popul" runat="server" visible="false">
   <asp:Label ID="lbl_rdb_study_popul" runat="server"
              CssClass="questions"
              Text="2.Select your study subjects">
   </asp:Label>

   <asp:RadioButtonList ID="rdb_study_popul" runat="server"
                        AutoPostBack="True" 
                        OnSelectedIndexChanged="rdb_study_popul_SelectedIndexChanged">
     <asp:ListItem>Individuals</asp:ListItem>
     <asp:ListItem>Population</asp:ListItem>
   </asp:RadioButtonList>

   <asp:RequiredFieldValidator ID="RequiredFieldValidator5" runat="server" 
                                ControlToValidate="rdb_study_popul"
                                Display="Dynamic" 
                                ErrorMessage="Study Subject is required" 
                                ValidationGroup="StudySubject">
   </asp:RequiredFieldValidator>
                        </div>
                    </td>
                </tr>
                <tr>
        <td>
    <asp:Button ID="btn_s_section" runat="server"  
                OnClick="btn_studysubject_section_Click"
                Text="Next" ValidationGroup="StudySubject"
                Visible="false" />
       </td>

Answer 1

You should add a validating group to the RadioButtonList definition too.

   <asp:RadioButtonList ID="rdb_study_popul" runat="server"
                        AutoPostBack="True" 
                        OnSelectedIndexChanged="rdb_study_popul_SelectedIndexChanged"
                        ValidationGroup="StudySubject">