DropDown Event won't fire

<asp:DropDownList AutoPostBack="true" ID="rating1"  runat="server" Width="60px"
                      onselectedindexchanged="rating1_SelectedIndexChanged">
                      <asp:ListItem>NA</asp:ListItem>
                      <asp:ListItem>1</asp:ListItem>
                      <asp:ListItem>1.5</asp:ListItem>
                      <asp:ListItem>2</asp:ListItem>
                      <asp:ListItem>2.5</asp:ListItem>
                      <asp:ListItem>3</asp:ListItem>
                      <asp:ListItem>3.5</asp:ListItem>
                      <asp:ListItem>4</asp:ListItem>
                      <asp:ListItem>4.5</asp:ListItem>
                      <asp:ListItem>5</asp:ListItem>
                  </asp:DropDownList>

protected void rating1_SelectedIndexChanged(object sender, EventArgs e)
  {
      float value = float.Parse(rating1.Text);
      if (value <= 3)
      {
          rating1.BackColor = System.Drawing.ColorTranslator.FromHtml("#FFFF66");
      }
      else
      {
          rating1.BackColor = System.Drawing.ColorTranslator.FromHtml("#FF3300");
      }
  }