ASP.NET Required Field Validator not working

You have to define the Validation Group Of your Textbox too....to make it work

   <asp:TextBox ID="txtTimeSlotGroupName" runat="server" 
        AutoPostBack="false" ValidationGroup="TimeSlot" ClientIDMode="Static"></asp:TextBox>

Even I was facing the same issue. Kindly check if any javascript are present on your page. Irrespective of above make use of Page.Validate() method and if(Page.IsValid) in your code. This will automatically force your validation controls and issue will be solved

Remove InitialValue="0" from the RequiredFieldValidator tag, it is not required when you are validating the textbox.