Android Question FTP UploadProgress does not fire for every file

'This function uploads the files (stored in the list) to the server base'
Public Sub CaricaFoto
    Log(filenamelist)
   
    currentTotalFiles = filenamelist.Size
    For Each filename In filenamelist
        currentFileSize = File.Size(File.DirInternal & $"/${Starter.FileManager1.NomeCartella}"$, filename)
       
        Dim sf As Object = FTP2.UploadFile(File.DirInternal & $"/${Starter.FileManager1.NomeCartella}"$, filename, False, $"${path}/${filename}"$)
        Wait For (sf) ftp2_UploadCompleted (ServerPath As String, Success As Boolean)
        If Success Then
            LogColor("File Caricato", Colors.Green)
            currentDoneFiles = currentDoneFiles + 1
            'File.Delete(File.DirInternal & $"/${Starter.FileManager1.NomeCartella}"$, filename)
           
            AggiornaProgress(Array As Object (0, currentDoneFiles, currentTotalFiles))
           
            If currentDoneFiles == currentTotalFiles Then 'fine trasferimento
                currentTotalFiles = 0
                currentDoneFiles = 0
                currentFileSize = 0
               
                Log("TRASFERIMENTO COMPLETATO")
                FTP2.Close
            End If
           
            Sleep(0)
        End If
    Next
End Sub

Private Sub ftp2_UploadProgress (ServerPath As String, TotalUploaded As Long, Total As Long)
    If (TotalUploaded/currentFileSize)*100 > lastProgValue + 5 Then 'if incremented more than 5, i print
        lastProgValue = (TotalUploaded/currentFileSize)*100
        Log($"Filesize: ${currentFileSize}, Progress: ${lastProgValue}"$)
        'AggiornaProgress(Array As Object (lastProgValue, currentDoneFiles, currentTotalFiles))

    End If
End Sub

'This function update the UI'
Public Sub AggiornaProgress(Data() As Object)
    'crcProgress.Value = Data(0) 'the value'
    'lblCountingFile.text = $"${Data(1)}/${Data(2)}"$ 'uploadedsize/totalsize'
    Sleep(10)
End Sub