ScriptManager.RegisterClientScriptBlock Not Working

Well I'm confused about StartupScript and RegisterClientScriptBlock with ScriptManager / ClientScript.

This part of code is not working for me, I mean the Javascript alerts are not showing. I'm using it within UpdatePanel.

Can anyone please help?

//Make From & To Equal for 1 Day
            if (txtTo.Text == "")
                txtTo.Text = txtFrom.Text;

            if (DateTime.Parse(txtTo.Text) >= DateTime.Parse(txtFrom.Text))
            {
                Connection con = new Connection(ConfigurationManager.ConnectionStrings["IPEConStr"].ConnectionString);

                //Get Mobile No Of User
                DataTable dtUser = new DataTable();
                dtUser = con.GetRecordSetByQry("SELECT [Mobile] FROM tblUser WHERE UserID = " + Session["UserID"].ToString()).Tables[0];

                if (CheckLeaveBalance())
                {
                    Leave lv = new Leave();
                    lv.UserID = Session["UserID"].ToString();
                    lv.From_Date = DateTime.Parse(txtFrom.Text);
                    lv.To_Date = DateTime.Parse(txtTo.Text);
                    lv.Leave_Type = ddlLeaveType.SelectedItem.Value;
                    lv.Status = "Pending";

                    if (con.Insert(lv) > 0)
                    {
                        string script = @"alert('Successfully Applied For Leave');";
                        ScriptManager.RegisterClientScriptBlock(UpdatePanel1, UpdatePanel1.GetType(), "SuccessLeave", script, true);

                        //Send SMS
                        string SMSString = "Your application for Casual Leave dated From " + txtFrom.Text + " To " + txtTo.Text + " is placed successfully subject to Approval by your Line Manager.\nRegards,\nIPE Global";
                        SMS sms = new SMS(dtUser.Rows[0]["Mobile"].ToString(), SMSString);
                    }
                }
                else
                {
                    string script = @"alert('You Don't Have Sufficient Leave Balance.');";
                    ScriptManager.RegisterClientScriptBlock(UpdatePanel1, UpdatePanel1.GetType(), "NoLvBal", script, true);
                }
            }
            else
            {
                string script = @"alert('To Date Can't Be Lesser Than From Date');";
                ScriptManager.RegisterClientScriptBlock(UpdatePanel1, UpdatePanel1.GetType(), "NoLvBal", script, true);
            }