Drop down list's SelectedIndexChanged event does not fire

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C#

ASP.NET

In my application,I call method at load time which fill college name into drop down list of college name.When I select one of the college,I want the faculties of corresponding that college from database.But SelectedIndexChanged event of drop down list of College name does not fire.Please give me a solution.
Thank You.
The code written by me is as follow.

protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack)
            {
                fillcollegename();
            }

        }

void fillcollegename()
    {
        objsqlconnection = new SqlConnection();
        objsqlconnection.ConnectionString = strconnectionstring;
        objsqlconnection.Open();
        try
        {

            string strquery1 = "select * from tblCollege_Info";

            objsqlcommand = objsqlconnection.CreateCommand();
            objsqlcommand.CommandText = strquery1;
            SqlDataAdapter a = new SqlDataAdapter(objsqlcommand);
            DataTable table = new DataTable();
            a.Fill(table);
            String[] CollegeNames = new String[table.Rows.Count];
            int i = 0;
            foreach (DataRow row in table.Rows)
            {
                CollegeNames[i] = row["CollegeName"].ToString();
                i++;
            }

        ddlCollegenm.DataSource = CollegeNames;
        ddlCollegenm.DataBind();
        }
        catch
        {
        }


        objsqlconnection.Close();
    }

protected void ddlCollegenm_SelectedIndexChanged(object sender, EventArgs e)
        {
            objsqlconnection = new SqlConnection();
            objsqlconnection.ConnectionString = strconnectionstring;
            objsqlconnection.Open();
            string strquery2 = "select CollegeId from tblCollege_Info where (CollegeName='" + ddlCollegenm.SelectedValue.ToString() + "')";
            objsqlcommand = new SqlCommand(strquery2, objsqlconnection);
            SqlDataReader objsqldatareader = objsqlcommand.ExecuteReader();
            if(objsqldatareader.Read())
            {
                collegeid = objsqldatareader[0].ToString();
            }
            objsqldatareader.Close();
            objsqlconnection.Close();
            fillfacultyname();
        }

void fillfacultyname()
        {
            objsqlconnection = new SqlConnection();
            objsqlconnection.ConnectionString = strconnectionstring;
            try
            {
                objsqlconnection.Open();
                string strquery1 = "select FacultyName from tblFaculty_Info where (CollegeId='" + collegeid + "')";
                objsqlcommand = objsqlconnection.CreateCommand();
                objsqlcommand.CommandText = strquery1;
                SqlDataAdapter a = new SqlDataAdapter(objsqlcommand);
                DataTable table = new DataTable();
                a.Fill(table);
                String[] FacultyNames = new String[table.Rows.Count];
                int i = 0;
                foreach (DataRow row in table.Rows)
                {
                    FacultyNames[i] = row["FacultyName"].ToString();
                    i++;
                }
                ddlfacultynm.DataSource = FacultyNames;
                ddlfacultynm.DataBind();
            }
            catch
            {
            }
            finally
            {
                objsqlconnection.Close();
            }
            }