Problem in cursor position lost in ASP.net

I am generating UserName based on FirstName.LastName after that I am hitting the database and checking if this username (Like AAA.BBB) exists or not if not then I am generating email address, if yes then it is thorowing error which is correct. And after this when I changed the username (Like AAA.BBBC) and hit enter or tab then again it should check the database and generate the email but it is not doing any thing means I think I am missing the cursor control.
Any idea?
 My code is: -
.aspx
<th>Username </th>
  <td>
   <asp:TextBox ID="txtUsernameB" runat="server" CssClass="txtRequired" MaxLength="20"
  TabIndex="4" AutoPostBack="true" ValidationGroup="vUserDetailsB" Width="243px" >
  </asp:TextBox>
<asp:RegularExpressionValidator ID="RegularExpressionValida ...