Cannot be show Processing dialog (PxProcessing dialog) for the newly created button on page Process Order

I have created a “Process Blanket Lines” Button on the Process Order page

I want to click the “Process Blanket Lines” button to display the Show Processing Dialog 
like so:

to monitor code behavior in function GenerateNewLineItems. However when I click Process Blank Lines it still doesn't show the Processing Dialog.

Here is my code:

using System;
using System.Collections;
using System.Collections.Generic;
using PX.Data;
using PX.Objects.CN.Subcontracts.AP.CacheExtensions;
using PX.Objects.SO;

public class SOCreateShipment_Extension : PXGraphExtension<PX.Objects.SO.SOCreateShipment>
{

    public PXAction<SOOrderFilter> processCustomization;
    [PXProcessButton(CommitChanges = true)]
    [PXUIField(DisplayName = "Process Blanket Lines", Visible = true)]
    protected virtual IEnumerable ProcessCustomization(PXAdapter adapter)
    {
        SOOrderFilter filter = Base.Filter.Current as SOOrderFilter;
        if (filter != null && filter.Action == "SO301000$addBlanketLine")
        {
            List<SOOrder> ordersToProcess = new List<SOOrder>();

            foreach (SOOrder order in Base.Caches[typeof(SOOrder)].Updated)
            {
                if (order.Selected == true)
                {
                    ordersToProcess.Add(order);
                }
            }

            foreach (SOOrder order in ordersToProcess)
            {
                ProcessOrder(order); 
            }

            return adapter.Get();
        }

        return adapter.Get();
    }

    private void ProcessOrder(SOOrder order)
    {
        try
        {
            PXProcessing<SOOrder>.SetCurrentItem(order);
            var filePath = PXCache<SOOrderFilter>.GetExtension<SOOrderFilterExt>(Base.Filter.Current)?.UsrNextSchShipDate;

            if (filePath != null)
            {
                GenerateNewLineItems(order, filePath);
                PXProcessing<SOOrder>.SetProcessed(); 
            }
            else
            {
                PXProcessing<SOOrder>.SetWarning("There is no Next Sch Ship Date yet"); 
            }
        }
        catch (Exception ex)
        {
            PXProcessing<SOOrder>.SetError(ex); 
        }
    }

    private void GenerateNewLineItems(SOOrder parentOrder, DateTime? nextSchShipDate)
    {
        PXTrace.WriteInformation("Generating new line items...");
        using (PXTransactionScope ts = new PXTransactionScope())
        {
            SOOrderEntry orderEntryGraph = PXGraph.CreateInstance<SOOrderEntry>();
            // logic customization
            orderEntryGraph.Save.Press();
            ts.Complete();
        }
    }
}

I used:
PXProcessing<SOOrder>.SetProcessed();
PXProcessing<SOOrder>.SetWarning();
PXProcessing<SOOrder>.SetError();

However it still doesn't show the Processing Dialog

Please help me see which part of my code is missing or incorrect?