Custom validator not working server side

Hi folks...a truly frustrating problem!!

Have a custom validator that just does not call the server validation function but it does call a clientside function if i add the tag.  To check it was not something i had done to the control or layout i added a data-type check as another validator immediately under:

<asp:CustomValidator ID="CustomValidatorPO" runat="server" ErrorMessage="If specified, it must be numeric and between 5 and 6 digits in length." ControlToValidate="txtPreviousOrder"
                                 Text="If specified, it must be numeric" OnServerValidate="CustomValidatorPO_ServerValidate" Display="Dynamic" SetFocusOnError="true" CssClass="validator"></asp:CustomValidator><asp:CompareValidator ID="CompareValidatorPO" runat="server" ControlToValidate="txtPreviousOrder"
                            ErrorMessage="Enter Numerical Value" Operator="DataTypeCheck" Type="Double" ValidationGroup="submitquote"
                            SetFocusOnError="true" CssClass="validator" Display="Dynamic" />

The data-type validator works fine but the custom validator procedure never calls the server side validator (i put a break point in and it never gets hit), the code which for reference is:

        protected void CustomValidatorPO_ServerValidate(object source, System.Web.UI.WebControls.ServerValidateEventArgs args)
        {
            //This is base class property used for debug
            //ErrMSG = "";

            //check for zero length first
            string sOrderNumber = args.Value.ToString();

            if (sOrderNumber.Length == 0)
            {
                args.IsValid = true;
                return;
            }

            //check if numeric and correct length
            try
            {
                int OrderNumber = int.Parse(args.Value);

                if (OrderNumber > 0 && (OrderNumber.ToString().Length > 5 && OrderNumber.ToString().Length <= 6))
                {
                    args.IsValid = true;
                }
                else
                {
                    ErrMSG = "If specified, it must be numeric and between 5 and 6 digits in length.";
                    args.IsValid = false;
                }
            }
            catch (Exception Ex)
            {
                ErrMSG = "Invalid value - unable to interpret as numeric code: "+Ex.Message;
                args.IsValid = false;
            }
        }  

If i comment the function out it complains it is not there so why is it not executing??