Sqldatasource is not updating on postback

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ASP.NET

i want to make a comment page. i have stored comments in database, it is getting stored but my asp:repeater is not updating after postback. please help.
this is my code:

<tr>
<asp:Repeater ID="repeater1" runat="server" DataSourceID="SqlDataSource1" >
<ItemTemplate>
<tr>
    <td>
        <%# Eval("Comment") %>
        <br />
    </td>


    <td align="right"> By: <%# Eval("UserName") %>
    <br />
    </td>
</tr>
</ItemTemplate>
</asp:Repeater>

</tr>
<tr>
<td><asp:TextBox ID="txtComment" runat="server" Height="110px" MaxLength="35565"
        Width="752px" TextMode="MultiLine"></asp:TextBox></td>
</tr>
<tr>
    <td align="right">
    <asp:Button ID="btnPost" runat="server" Text="Post" Visible="false"
            onclick="btnPost_Click" />
    &nbsp;&nbsp;&nbsp;&nbsp;
    <asp:Button ID="btnCmmnt" runat="server" Text="Comment" onclick="btnCmmnt_Click"/>

    </td>
</tr>
<asp:SqlDataSource ID="SqlDataSource1" runat="server"
        ConnectionString="<%$ ConnectionStrings:WebCommentConnectionString %>"
        SelectCommand="SELECT * FROM [Comment] where [Post_ID]=12"></asp:SqlDataSource>
</table>