Required Field Validator Not Working

I have a required field validator for a text box, however, it only works if I a use a ValidationSummary. If I remove the ValidationSummary then the page is posted back and no validation is done at all.
Here is the validator and text box. Can anyone tell me why the validation is not happening?


<asp:RequiredFieldValidator ID="RequiredFieldValidator_txtMiles" ControlToValidate="txtMiles" EnableClientScript="true" Display="Dynamic" SetFocusOnError="true" Text="Miles is a required field" Enabled="true" runat="server"></asp:RequiredFieldValidator> <asp:TextBox ID="txtMiles" runat="server" Height="20px" Width="74px"></asp:TextBox>   <asp:TextBox ID="txtTime" runat="server" Height="20px" Width=" ...