Multiple validationgroup in a single page - Not working

I have two sets of validation group in a single page. When I click on the button(s), it displays the error message ("required field"). However, it still processes the button click function as if the form is valid.

//First Group<asp:TextBox ID="txt_PostMessage_URL" CssClass="textbox" Width="100%" runat="server" Text="Type your message.." onfocus="if(this.value=='Type your message..')this.value='';" onblur="if(this.value=='')this.value='Type your message..'" ValidationGroup="LinkMessage"></asp:TextBox><asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server" ControlToValidate="txt_PostMessage_URL" CssClass="failureMessage" ErrorMessage="RequiredFieldValidator" InitialValue="Type your message.." ValidationGroup="LinkMessage" Display="Dynamic" SetFocusOnError="True">* Please enter your message.</asp:RequiredFieldValidator><asp:Button ID="btn_Cancel_URL" runat="server" CssClass="cancel" OnClick="btn_Cancel_Video_Click"    Text="Cancel" ValidationGroup="LinkMessage" /><asp:Button ID="btn_Submit_URL" runat="server" CssClass="bluebutton" OnClick="btn_SubmitLink_Click" Text="Post" Width="70px" ValidationGroup="LinkMessage" />

//Second group

<asp:TextBox ID="txt_PostMessage_Video" CssClass="textbox" Width="100%" runat="server" Text="Type your message.." onfocus="if(this.value=='Type your message..')this.value='';" onblur="if(this.value=='')this.value='Type your message..'" ValidationGroup="VideoMessage"></asp:TextBox><asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server" ControlToValidate="txt_PostMessage_Video" CssClass="failureMessage" ErrorMessage="RequiredFieldValidator" InitialValue="Type your message.." ValidationGroup="VideoMessage" Display="Dynamic" SetFocusOnError="True">* Please enter your message.</asp:RequiredFieldValidator><asp:Button ID="btn_Cancel_Video" runat="server" CssClass="cancel" OnClick="btn_Cancel_Video_Click"    Text="Cancel" ValidationGroup="VideoMessage" /><asp:Button ID="btn_Submit_Video" runat="server" CssClass="bluebutton" OnClick="btn_SubmitLink_Click" Text="Post" Width="70px" ValidationGroup="VideoMessage" />

I used multiple validationgroups in a single page a lot. But, do not get it why it does not work this time. What could be the reason?