How requiredfieldvalidator only check one textbox in a aspx page with 6 textbox?

I have 6 user control ascx pages, there is a textbox and submit button in each of them. I have a aspx page to include all those 6 user control pages.
I have a requiredfieldvalidator with onsubmit to check empty of all textbox. If the text box is empty and user clicks submit, it will be prompted.

Here is the aspx code

<asp:textbox id="text1" runat="server"></asp:textbox>
<asp:requiredfieldvalidator id="RequiredText1" runat="server" EnableViewState="False" ControlToValidate="text1" Display="None" ErrorMessage="Please input 1"></asp:requiredfieldvalidator>
<asp:validationsummary id="ValidationSummary" runat="server" EnableViewState="False" ShowMessageBox="True" ShowSummary="False"></asp:validationsummary>
<asp:button id="button1" runat="server" text="submit& ...