Textbox inside the Gridview doesnt raise an event

Question

Hi friends,
i have a textbox named txtrateperUnit inside Gridview and i have used Itemtemplate on .Aspx page like below
<ItemTemplate>
  <asp:TextBox ID="txtrateperUnit" runat="server" ontextchanged="txtrateperUnit_TextChanged" AutoPostBack="true"></asp:TextBox>
</ItemTemplate>
and on Code behind i have raised the event:
protected void txtrateperUnit_TextChanged(object sender, EventArgs e)
  {
string a=a+10;
  }
but when i add some value in the textbox in runtime the event doesnt get raised...
 
what could be the Probs.
 
Pl reply
 
regards,
aamir