Interesting... See if this works:

    For Ws = 1 To 2
        With Worksheets(Ws)
            .Cells(4, MonthColumn) = "Total"
            For i = 7 To 16
                .Cells(i, MonthColumn).FormulaR1C1 = "=SUM(RC2:RC[-1])"
            Next i
        End With
    Next Ws
If not, is it possible you could post up a copy of your workbook?