Page.IsValid always True

Hi,
Am spending hours trying to get to the bottom of this.  I have a user control within a hidden panel which is made visible after a postback (my first button checks a validation for a patient number).  The panel has a from and a second submit button that will send data back - at this point I do further validation...
I have a RequiredFieldValidator on a text box but when my button is clicked the Page.IsValid always returns True, even if the field is not filled in.
here is the form snippit:
&lt;asp:TextBox ID="TextBox3" runat="server" ValidationGroup="ValGroup1"&gt;</asp:TextBox> <asp:RequiredFieldValidator ID="ReqV1" ValidationGroup="ValGroup1" runat="server" EnableClient ...