Как мне решить проблему с случае изменения dropdownlist для обжига один раз, затем пропускает одно событие изменить, затем срабатывает то пропускает(альтернативное событие change стрельбы)... И так далее ?

protected void ddlCollection_SelectedIndexChanged(object sender, EventArgs e)
      {

          ddlSerial.SelectedIndex = -1;
          hdfQDSID.Value = "";
          txtQuantity.Text = "";
          lblTypeT.Text = "";
          lblQds.Text = "";
          lnkWHStock.Text = "";
          lblPieces.Text = "";
          lblUnit.Text = "";


          string CollectionID = ddlCollection.SelectedValue.ToString();

          if (ddlCollection.SelectedValue.ToString() != "0")
          {

              DataTable dt = new DataTable();
              DataSet ds = objBAL.getGridCollectionSrNo(CollectionID.ToString());

              if (ds.Tables[1].Rows[0]["COLLECTION_STATUS"].ToString() == "DISCONTINUED")
              {
                  ScriptManager.RegisterStartupScript(Page, GetType(), "CollectionDscontiAler", "<script>alert('This Collection has been discontinued.');</script>", false);
              }


              dt = ds.Tables[0];
              ddlSerial.DataSource = dt;
              ddlSerial.DataTextField = "SR_NO";
              ddlSerial.DataValueField = "SR_NO";
              ddlSerial.DataBind();
              ddlSerial.Items.Insert(0, new System.Web.UI.WebControls.ListItem("Select Sr No", "0"));
              ddlSerial.SelectedIndex = 0;
          }
      }

protected void ddlSerial_SelectedIndexChanged(object sender, EventArgs e)
      {
          hdfQDSID.Value = "";
          txtQuantity.Text = "";
          lblTypeT.Text = "";
          lblQds.Text = "";
          lnkWHStock.Text = "";
          lblPieces.Text = "";
          lblUnit.Text = "";

          string QDSName = string.Empty;
          string QDSID = string.Empty;
          string TypeOfFabric = string.Empty;

          string CollectionID = ddlCollection.SelectedValue.ToString();
          string SrNo = ddlSerial.SelectedValue.ToString();


          if (SrNo != "0" && CollectionID != "0")
          {

              DataTable dt = new DataTable();
              DataSet dsQdsName = objBAL.getGridCollectionQDS(CollectionID, SrNo);  //uspB2BCustomerPortal_uspGetCollectionQDS
              dt = dsQdsName.Tables[0];

              if (dt.Rows.Count > 0)
              {
                  QDSName = dt.Rows[0]["QDSName"].ToString();
                  QDSID = dt.Rows[0]["QDS_ID"].ToString();
                  hdfMultipleOf.Value = dt.Rows[0]["MULTIPLE_OF"].ToString();
                  if (dsQdsName.Tables[1].Rows[0]["PRODUCT_STATUS"].ToString() == "DISCONTINUED")
                  {
                      ScriptManager.RegisterStartupScript(Page, GetType(), "QDSDiscontinuedAlertU", "<script>alert('This QDS has been discontinued.');</script>",true);
                  }

              }

              TypeOfFabric = lblOrderTypeDesription.Text.ToString();
              if (QDSName != "")
              {
                  lblQds.Text = QDSName.ToString();
                  hdfQDSID.Value = QDSID.ToString();

                  //bool result = CheckQDSOrderForCustomer();

                  //if (result)
                  //{


                  if (QDSID != "")
                  {
                      //Code for WHStock & No of Pcs.
                      B2B_Order_Master_SAP_BAL obj_BAL = new B2B_Order_Master_SAP_BAL();
                      DataSet dsWHStockinfo = obj_BAL.getWHStock(QDSID, hfCustomerId.Value.ToString());   //usp_B2BCustomerPortal_getCreateOrderWHStock
                      lnkWHStock.Text = dsWHStockinfo.Tables[0].Rows[0]["WHStock"].ToString();
                      lblPieces.Text = dsWHStockinfo.Tables[0].Rows[0]["No_Of_Pieces"].ToString();

                      //if (dsWHStockinfo.Tables[0].Rows[0]["No_Of_Pieces"].ToString() != "")
                      //{
                      //    lblPieces.ForeColor = System.Drawing.Color.Red;
                      //}
                  }

                  if (TypeOfFabric == "Fabric Order")
                  {
                      lblUnit.Text = "Mtrs.";
                  }
                  else if (TypeOfFabric == "Non-Fabric Order" || TypeOfFabric == "Catalogue Order")
                  {
                      lblUnit.Text = "Pcs";
                  }

                  //}
                  //else
                  //{
                  //    ScriptManager.RegisterStartupScript(Page, GetType(), "Last3DaysTwo2", "<script>AlertUser();</script>", false);
                  //    YesAlertUser.Focus();
                  //}
              }
              else
              {
                  lblQds.Text = "QDS Not Found.";
              }
          }
      }

<asp:UpdatePanel ID="UpdatePanel1" UpdateMode="Conditional" runat="server">
           <contenttemplate>
</contenttemplate>
           <triggers>
               <asp:AsyncPostBackTrigger ControlID="btnShowInfo" EventName="Click" />
               <asp:AsyncPostBackTrigger ControlID="btnAddItems" EventName="Click" />
               <asp:AsyncPostBackTrigger ControlID="ddlSerial" EventName="SelectedIndexChanged" />
           </triggers>

       <asp:UpdateProgress ID="updPanelProgress" runat="server" AssociatedUpdatePanelID="UpdatePanel1">
           <progresstemplate>
               <div class="pre-loader"></div>
           </progresstemplate>

<table><tbody><tr><td>
<asp:DropDownList ID="ddlSerial" runat="server" onchange="SetScroll();" AutoPostBack="true" OnSelectedIndexChanged="ddlSerial_SelectedIndexChanged">

</td></tr></tbody></table>