Excel Vba Loop , not running properly

Option Explicit

Private Sub validar()

    Dim folha           As Long                 ' sheet number
    Dim src             As Workbook             ' a workbook from which alerts are being read
    Dim lastRow         As Long                 ' last row with content in a worksheet
    Dim alertNum        As Long                 ' Alert number being updated
    Dim k               As Long                 ' counter
    Dim ref             As String               ' reference of the alert 
    Dim nac             As Long                 ' nac ?
    Dim npc             As Long                 ' npc ?

Application.ScreenUpdating = False
On Error GoTo ErrHandler

    folha = CLng(estadoform.Label1.Caption)
    With ThisWorkbook.Worksheets(folha)
        lastRow = .Range("A65536").End(xlUp).Row
        alertNum = .Cells(lastRow, 6)
    End With

    ' ABRIR EXCEL
    Set src = Workbooks.Open("U:\Mecânica\Produção\OEE\OEE ( FULL LOG )\OEEalerta.xlsx", True, False)
    With src.Sheets("alerta")
        lastRow = .Range(" A10000").End(xlUp).Row
        For k = 1 To lastRow
            ref = .Cells(k, 2)
            npc = .Cells(k, 4)
            nac = .Cells(k, 5)

            If ref = alertNum And (nac < npc) Then .Cells(k, 5) = nac + 1    ' update where the filter conditions are met
        Next k
    End With

ErrHandler:
    Application.EnableEvents = True
    Application.ScreenUpdating = True
    Application.DisplayAlerts = False 'IT WORKS TO DISABLE ALERT PROMPT
    'SAVES FILE USING THE VARIABLE BOOKNAME AS FILENAME
    src.Save
    Application.DisplayAlerts = True 'RESETS DISPLAY ALERTS
    ' CLOSE THE SOURCE FILE.
    src.Close False             ' FALSE - DON'T SAVE THE SOURCE FILE (since it has already been saved)
    Set src = Nothing

End Sub