Event not firing.

I have a GridView(GV) and inside the GV I have a databound column. The column has a Checkbox in it and I have tried to wire up the OnCheckedChanged event to a code block in the code behind file. The event signature is correct I just can't get it to fire. The GV is inside of a Update panel. Any ideas?

Class _Default : Inherits Page

  Protected Sub cb_cc(sender As Object, e As EventArgs)
   'do stuff
  End sub
... 

<asp:CheckBox ID="cb" runat="server" Checked="false" OnCheckedChanged="cb_cc" />