FindControl cannot find checkbox in c#

I have checkboxes created in repeater:

    <asp:Repeater ID="companyRepeater" runat="server" >
  <ItemTemplate>

<tr class = "DGItemStyle"  id="myresultsRow1" runat="server" style='<%# SetBoxVisibilityReverse(Eval("compnName1").ToString()) %>'> 
    <td  style="padding:0.5em;"><%#Eval("compnAddress1") %></td> 
    .....
    <td><asp:CheckBox id="foo" runat="server"  AutoPostBack="True" name="foo" /></td>
</tr>

I want to find out which checkbox is currently selected by user, so in the codebehind I do:

CheckBox chkbox_All = FindControl("foo") as CheckBox;
if (chkbox_All != null)
{
   if (!chkbox_All.Checked)
   {
       Response.Write("No checked");
   }
   else
   {
       var IDs = chkbox_All.ClientID;
       Response.Write("ID here...");
   }
}

But what I found is that the chkbox_All is always null no matter how many checkboxes are selected. Why is it so and how could I find which checkbox is checked in this case ?