Excel VBA SUM devuelve cero o error

He intentado simplificar una publicación anterior centrándome en una sola pregunta relacionada con Excel.WorksheetFunction.Sum. Claramente no sé cómo usar esta función, ya que sigo obteniendo Cero por un resultado o arrojando errores.

Simplemente no puedo hacer que esto funcione y lo he estado trabajando durante tres días (literalmente) y he realizado casi cien pruebas distintas. Aunque entiendo bien Access VBA, soy autodidacta y creo que me faltan algunos conceptos clave y eso me está matando en Excel.

Mi código a continuación es de prueba / depuración que he realizado y la documentación explica los resultados para cada versión distinta. El sub llama a la función (que es donde están los problemas).

Algunos puntos:

1. PUEDO obtener una SUMA en la hoja de trabajo cuando estoy usando Excel manualmente.

   1a. Los números en el Rango requerido no siempre son contiguos, pero si reduzco el rango a solo números contiguos, todavía NO funciona.

2. Estoy escribiendo esto en un módulo de acceso, ya que es parte de una aplicación de acceso (tratando de automatizar la importación de datos desde una hoja de cálculo).

3. Se ha implicado que mi trabajo anterior no estaba "totalmente calificado", por lo que he creado esto usando un With Bloc. Sin embargo, es probable que NO esté haciendo esto correctamente.

Cualquier orientación sería muy apreciada, especialmente si pudieras explicar misericordiosamente qué conceptos me estoy perdiendo aquí.

Public Function fnImportFileProcessFilePrep2(intClientId As Integer, intEventId As Long, strExcelFileName As String, _
strActiveSheet As String, strQASumColumn As String)
On Error GoTo HandleError

Dim intLastCol As Long
Dim intLastRow As Long
Dim intNextCol As Long
Dim intRecordCount As Long

Dim varSumExcelColumns As Variant
Dim strSUMRange As String
Dim strAddColumnLabel As String
Dim dblSum As Double

Dim rgUseRange As Range
Dim rgSUMRange As Range

Dim strFileName As String

Dim oXLApp As Excel.Application       'Declare the object variables
Dim oXLBook As Excel.Workbook
Dim oXLSheet As Excel.Worksheet

Set oXLApp = New Excel.Application      'Create a new instance of Excel
Set oXLBook = oXLApp.Workbooks.Open(strExcelFileName) 'Open the existing workbook
Set oXLSheet = oXLBook.Worksheets(strActiveSheet)  'Work with the input worksheet


  oXLSheet.Activate                   'Activate the Worksheet
  oXLApp.Visible = True               'Show it to the user
  oXLApp.UserControl = True


    With oXLSheet
        ' Replace ALL "(null)" cells - THIS WORKS!!!
            .Cells.Replace What:="(null)", _
            Replacement:="", _
            LookAt:=xlWhole, _
            SearchOrder:=xlByRows, _
            MatchCase:=False


        'BOTH LastRow and LastCol WORK
        'Get Last Row & Record Count

            intLastRow = oXLSheet.Cells.Find("*", SearchOrder:=xlByRows, SearchDirection:=xlPrevious).Row  'This Works
            intRecordCount = intLastRow - 1

        'Get Last Column
            intLastCol = oXLSheet.Cells.Find("*", SearchOrder:=xlByColumns, SearchDirection:=xlPrevious).Column 'This Works
            intNextCol = intLastCol + 1





    'Get SUM of Column strQASumColumn for use in QA

        'NONE of the following work.  Note that if I use Select it's for testing so that I can look at Open Excel Sheet and see the Select Range is correct

            strSUMRange = strQASumColumn & "2:" & strQASumColumn & intLastRow '  "M2:M2934"
            Set rgSUMRange = .Range(strSUMRange)
            'rgSUMRange.Select
            varSumExcelColumns = Excel.WorksheetFunction.Sum(rgSUMRange) 'Works BUT IS ZERO??
            dblSum = Excel.WorksheetFunction.Sum(rgSUMRange) 'Works but ZERO?
            varSumExcelColumns = Excel.WorksheetFunction.Sum(oXLSheet.Range(strSUMRange))  'Works but Zero



        'Try to use Cells
             Set rgSUMRange = .Range(.Cells(2, "M"), .Cells(intLastRow, "M"))
             rgSUMRange.Select
             varSumExcelColumns = Excel.WorksheetFunction.Sum(rgSUMRange) 'Works but Zero SUM

             Set rgSUMRange = .Range(.Cells(2, intNextCol), .Cells(intLastRow, intNextCol))
             varSumExcelColumns = Excel.WorksheetFunction.Sum(rgSUMRange) 'Works but Zero

             'Even Hard-coding the numbers does NOT work
             Set rgSUMRange = .Range(.Cells(2, 13), .Cells(2934, 13)) ' Returns Zero Again
             'rgSUMRange.Select  ' Does show the correct Range
             varSumExcelColumns = Excel.WorksheetFunction.Sum(rgSUMRange)

             'Still Zero if I use a smaller range that has contiguous numbers
             Set rgSUMRange = .Range(.Cells(3, 13), .Cells(7, 13)) ' Returns Zero Again
             rgSUMRange.Select
             varSumExcelColumns = Excel.WorksheetFunction.Sum(rgSUMRange)

        'All these approaches ERROR
            'varSumExcelColumns = Excel.WorksheetFunction.Sum(.Range(rgSUMRange)) 'Method Range of Object Worksheet Failed
            'varSumExcelColumns = Excel.oXLSheet.WorksheetFunction.Sum(.Range(rgSUMRange)) 'Won't compile
            ' varSumExcelColumns = Excel.WorksheetFunction.Sum("M2:M2934")  'Unable to get the SUM Property of the WorksheetFunction Class
            'varSumExcelColumns = Excel.WorksheetFunction.Sum(Range("M2:M2934")) 'Application defined or object defined error

            'dblSum = Excel.WorksheetFunction.Sum("M2:M100") 'ERROR:  Unable to get the SUM Property of the Worksheet function Class
            'dblSum = Excel.WorksheetFunction.Sum(Range("M2:M100")) 'Application defined or --- Error

            'varSumExcelColumns = Excel.WorksheetFunction.Sum(Worksheets(strActiveSheet).Range("M2", "M7")) 'ERROR Application Defined or Object Defined Error




      'Go to EMPTY Range next to the Last Column

             varSumExcelColumns = Excel.WorksheetFunction.Sum(.Range(.Cells(2, intNextCol), .Cells(intLastRow, intNextCol)))  ' Works for SUM but is wrong Range
             'THE ABOVE ACTUALLY WORKS, BUT ONLY IF I GO TO OPEN SPREADSHEET AND MANUALLY ENTER NUMBERS INTO THE RANGE AREA ?????????

        'Since the above kinda worked, Try setting variables to a Range WITH Number data - Does NOT Work
             intNextCol = 13
             intLastRow = 7
             varSumExcelColumns = Excel.WorksheetFunction.Sum(.Range(.Cells(2, intNextCol), .Cells(intLastRow, intNextCol)))
             msgbox "SUM:  " & varSumExcelColumns


        'Test to see if I am still on the Correct Sheet - This WORKS
             Dim dblCellValue As Double
             dblCellValue = oXLSheet.Cells(2, 13).Value  'Works



    End With




Exit_Label:
    fnImportFileProcessFilePrep2 = varSumExcelColumns


    oXLBook.Close SaveChanges:=False  'SaveChanges:=True    'Save (and disconnect from) the Workbook



    oXLApp.Quit                         'Close (and disconnect from) Excel
    Set oXLSheet = Nothing               'Disconnect from all Excel objects (let the user take over)
    Set oXLBook = Nothing
    Set oXLApp = Nothing

Exit Function

HandleError:


    msgbox "Error During fnImportFileProcessFilePrep2: " & Err.Description

    Resume Next
End Function

Private Sub TestFilePrep()
Dim strFileNameAndPath As String
Dim strUseWorksheet As String
Dim intSUMColumn As Integer
Dim strSUMColumn As String
Dim strAddColumnLabel As String
Dim varAddColumnFixedValue As Variant

Dim dblSUMFromFunction As Double

strFileNameAndPath = "C:\Users\xxxxxxxWITH NULLS2.xlsx"
strUseWorksheet = "Sheet1"
intSUMColumn = 13
strSUMColumn = "M"
strAddColumnLabel = "SourceFile"
varAddColumnFixedValue = 77


dblSUMFromFunction = fnImportFileProcessFilePrep2(10, -3, strFileNameAndPath, _
strUseWorksheet, strSUMColumn)

End Sub