Need help in binding dropdownlist

I want to bind 2 dropdownlist, ddlC and ddlS upon selected to ddlM. However, ddlC codes can work, while the ddlS codes can't work. I face problems and do not know how to do.. Please help.

Thanks.


protected void ddlC_SelectedIndexChanged(object sender, EventArgs e)
{
ddlM.Items.Clear(); //upon selected c, refresh m
ddlM.Items.Add(new ListItem("All", "%"));
string strCon4 = Database.GetConStr();
SqlConnection sqlCon4 = new SqlConnection(strCon4);
try
{ //bind the M dropdownlist
string strSql4 = "SELECT DISTINCT NOM, C FROM M WHERE C = @c";
SqlCommand sqlCmd4 = new SqlCommand(strSql4, sqlCon4);
sqlCmd4.Parameters.AddWithValue("@c", ddlC.SelectedValue);
sqlCon4.Open();
SqlDataReader reader4 = sqlCmd4.ExecuteReader();
while (reader4.Read())
{
ddlM.Items.Add(new ListItem(reader4["NOM"].ToString()));
}
reader4.Close();
}
catch (Exception ex)
{
lblError.Text = ex.Message;
}
finally
{
sqlCon4.Close();
}

}


The codes on top can work. However, the bottom codes can't work. I don't know what went wrong..


protected void ddlS_SelectedIndexChanged(object sender, EventArgs e)
{

ddlS.Items.Clear();
ddlS.Items.Add(new ListItem("All", "%"));

string strCon5 = Database.GetConStr();
SqlConnection sqlCon5 = new SqlConnection(strCon5);
try
{
string strSql5 = "SELECT C, S FROM M WHERE S = @s";
SqlCommand sqlCmd5 = new SqlCommand(strSql5, sqlCon5);
sqlCmd5.Parameters.AddWithValue("@s", ddlS.SelectedValue);
sqlCon5.Open();
SqlDataReader reader5 = sqlCmd5.ExecuteReader();
while (reader5.Read())
{
ddlM.Items.Add(new ListItem(reader5["S"].ToString()));
}
reader5.Close();
}
catch (Exception ex)
{
lblError.Text = ex.Message;
}
finally
{
sqlCon5.Close();
}
}