RequiredFieldValidator doesn't always work?

On my page, in a panel, in a table (multiple rows and columns),  I have a number of text boxes.  If I tab from the textbox below, I understand that nothing has changed, and thus the ontextchanged is not called.  What I do not understand is why the requiredFieldValidator does not kick in and tell me that nothing was entered in the textbox.



<asp:TableRow ID="TableRow1" runat="server">
               <asp:TableCell ID="TableCell2" runat="server" style="text-align:left">
                 Name or Handle:
               </asp:TableCell>
               <asp:TableCell ID=" ...