Required Field Validation not working in asp.net

Required Field Validation not working in asp.net

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>