I have many cells using a formula I created in VBA (below).

When the workbook opens it asks me to allow macro, and I do. Then it asks
if I want to update, and I do. The cells remain un-updated, however, until I
double-click on them as if to edit, then hit enter. Only then does any one
cell update.

I have made sure the File-Options-Calculate-Autocalculate option has been
selected.
F9 has no effect.

Note that the worksheet "Mark P" is hidden in this workbook, and the values
in Mark P are coming from a worksheet in a different workbook. I have
confirmed that this part is working properly, though.

You help is seincerely appreciated.

Function MarkP(task, start, fin)
hours = 0
For Row = start To fin
Set curCell = Worksheets("Mark P").Cells(Row, 4)
Set valCell = Worksheets("Mark P").Cells(Row, 5)
If curCell.Value = task Then
hours = hours + valCell.Value
End If
Next Row
MarkP = hours

End Function