Web Forms :: Custom Validation Only Works For FileUpload Control

I have 2 custom validator controls on my page but for some strange reason the Custom Validator for FileUpload control works and the custom validator for TextBox control does not work. Here is the aspx code:

<asp:FileUpload ID="_fileCv" runat="server" Width="500px" /><br />
<asp:RequiredFieldValidator ID="_rfvFileCv" runat="server" ErrorMessage="Resume; " ControlToValidate="_fileCv" SetFocusOnError="True" ValidationGroup="APPLY">
<img src="images/mandatory.gif" alt="Required" /></asp:RequiredFieldValidator
<asp:CustomValidator ID="CustomValidator1" runat="server" ControlToValidate="_fileCv"
ErrorMessage="Max file size for documents is 4MB." OnServerValidate="CustomValidator1_ServerValidate" ValidationGroup="APPLY"></asp:CustomValidator>
<asp:TextBox ID="_txtHealthComments" runat="server" TextMode="MultiLine" Rows="5" Width="500" />
<asp:CustomValidator ID="CustomValidator2" runat="server" ControlToValidate="_txtHealthComments"
ErrorMessage="Kindly enter health Comments ." OnServerValidate="CustomValidator2_ValidateHealth" ValidationGroup="APPLY"></asp:CustomValidator>

Here is the code behind:
protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args) {
if (_fileCv.FileBytes.Length > 4194304) {
args.IsValid = false;
} else { args.IsValid = true; } }
protected void CustomValidator2_ValidateHealth(object source, ServerValidateEventArgs args) {
if (_rdoHealth.SelectedValue == "1" && _txtHealthComments.Text == "") {
args.IsValid = false; } else { args.IsValid = true; } }